RangeSelect/MultiSelect: Fix so that shift+arrow when landing on an item that doesn't hold multi-select data doesn't trigger an unbounded range-select that ends up selecting everything until the end.

RangeSelect/MultiSelect: WIP range-select (ref 1861) [rebased]

Version 1.71 + comments

Internals: Reworked RenderTextEllipsis() to satisfy what we need for table headers.

CollapsingHeader: Minor fix to align right side of frames (which is extruded past the Work/Contents rect) with clipping rectangle.

Log/Capture: Fixed BeginTabItem() label not being included in a text log/capture. ...

Extracted tab rendering code into a RenderTextEllipsis() function.

Internals: Added drawlist and color arg to RenderArrow(), RenderBullet(). Reordered args for RenderPixelEllipsis.

Synced/merged minor cruft from docking branch to minimize drift. AFAIK the only meaningful no-op change is that the call UpdateHoveredWindowAndCaptureFlags() was moved above UpdateMouseMovingNewFrame() to match what docking branch has been doing for a while.

Scrollbar overlap an extra WindowBorderSize amount on the left to make all distances consistent. Reverted to BorderSize not affecting work/contents rectangles. Scrollbar, Style: Changed default style.ScrollbarSize from 16 to 14.

Internals: Minor no-op tidying up toward solving the WindowPadding / WindowBorderSize / ScrollbarSize overlapping mess. ...

+ Demo: Use SetScrollY().

Internals: Renamed SizeContents to ContentSize, SizeContentsExplicit to ContentSizeExplicit. Tweaked Metrics->Show Rectangles functionality.

Window rectangles: Changed WorkRect to cover the whole region including scrolling (toward obsolete ContentsRegionRect) + using full WindowPadding*1 padding. ...

Tweaked InnerClipRect.
TreeNode, CollapsingHeader: Fixed highlight frame not covering horizontal area fully when using horizontal scrolling. (#2211, #2579)
TabBar: Fixed BeginTabBar() within a window with horizontal scrolling from creating a feedback loop with the horizontal contents size.
Columns: Fixed Columns() within a window with horizontal scrolling from not covering the full horizontal area (previously only worked with an explicit contents size). (#125)
Demo: Added demo code to test contentsrect/workrect

Clarified behavior of SetNextWindowContentSize(). Content size is defined as the size available after removal of WindowPadding on each sides. So SetNextWindowContentSize(ImVec2(100,100)) + auto-resize will always allow submitting a 100x100 item without creating a scrollbar, regarding of WindowPadding.The exact meaning of ContentSize for decorated windows was previously ill-defined.

Comments about obsoleted features version. Todo. Clarify tab bar initial offset (useful if we decide to remove the half-windowpadding clip margin).

Combo: Fixed rounding not applying with the ImGuiComboFlags_NoArrowButton flag. (#2606, #2607)

Internals: Window rectangles: Renaming of all rectangles toward their final form. Should be a no-op. Renamed GetWorkRectMax() to GetContentRegionMaxAbs(). Metrics shows SizeContents.

CollapsingHeader: When a close button is enabled, better clip the label to avoid overlap. (#600)

Internal: Renamed InnerMainRect to InnerVisibleRect. Printing coordinates in Metrics window.

ColorEdit: Fixed the color picker popup only displaying inputs as HSV instead of showing multiple options. (#2587, broken in 1.69 by #2384).

Style: Added style.WindowMenuButtonPosition (left/right, defaults to ImGuiDir_Left) to move the collapsing/docking button to the other side of the title bar.

Internal: CloseButton takes an upper-left corner + a size to be consistent with similar widgets.

Internals: Extracted some of the tab bar shrinking code into a ShrinkWidths() function so columns/table can use it.

Fixed InputScalar, InputScalarN, SliderScalarN, DragScalarN with non-visible label from inserting style.ItemInnerSpacing.x worth of trailing spacing.

Internals: Added InnerWorkRect equal to old InnerClipRect, added InnerWorkRectClipped actually clipped.

Fixed crash when appending with BeginMainMenuBar() more than once and no other window are showing. (#2567) + comments

Fixed GCC mem-access warnings (#2565) ...

+ using "if defined" more consistently for Clang.

Fixed ColorEdit breakage introduced by d3a387c (#2557, #1875, #2034)

Fixed InputFloatX, SliderFloatX, DragFloatX functions erroneously reporting IsItemEdited() multiple times when the text input doesn't match the formatted output value (e.g. input "1" shows "1.000"). It wasn't much of a problem because we typically use the return value instead of IsItemEdited() here. (#1875, #2034)

Internal: Renamed fields + minor tweaks (probably shallow break stack-layout pr, sorry!)

Internal: Removed GetNextItemWidth(), relying on ItemAdd or NextItemData.ClearFlags() to clear the width data. Amend 5078fa20 and undo some of its effects of imgui_widgets.cpp