Internal: Refactored Separator into SeparatorEx(), exposed ImGuiSeparatorFlags_SpanAllColumns in imgui_internal.h and support without. (#759) + misc comments

Columns: Fixed Separator from creating an extraneous draw command. Fixed Selectable with SpanAllColumns flag from creating an extraneous draw command. (#125)

Version 1.71 WIP + fixed minor typo

Version 1.70

Demo: Improved trees in columns demo. (#2136)

Internals: Rename GetContentRegionMaxScreen() -> GetWorkRectMax(). At this point this is mostly useful to facilitate merge of other branches.

Popups: Closes popup at the time of FocusWindow(). Fixes right-click from closing all popups instead of aiming at the hovered popup level (regression in 1.67's ae76a1fd).

Popups: Closing a popup restores the focused/nav window in place at the time of the popup opening, instead of restoring the window that was in the window stack at the time of the OpenPopup call. (#2517) ...

Among other things, this allows opening a popup while no window are focused, and pressing Escape to clear the focus again.

Internals: Added DataTypeApplyOp, DataTypeApplyOpFromText to imgui_internal.h

Internals: Renaming. Renamed ImGuiPopupRef to ImGuiPopupData for consistency and added constructor.

Internals: Exposed ImGuiDataTypeInfo, DataTypeGetInfo(), DataTypeFormatString(). Comments.

Internals: TempInputText: Rename InputScalarAsWidgetReplacement() -> TempInputTextScalar(), ScalarAsInputTextId -> TempInputTextId, small tidying up in affected functions.

Internals: Using more explicit PushOverrideID() helper + renamed equivalent internal tree helper.

Misc comments, internal renaming, added disable indentation option to Columns demo section.

Internals: ImHashStr() default parameter.

Added SetNextItemWidth() helper to avoid using PushItemWidth/PopItemWidth() for single items.

Added commentary about ContentRegion functions. Added internal GetContentRegionMaxScreen() to facilitate internal code at the moment.

Internals: Moved CalcItemSize next to CalcItemWidth, added comments to clarify their respective intent. Should have no side effect.

Internals: Columns: Tweaks, renaming. Metrics: Show rectangles for child windows. ...

Renamed SameLine() first parameter.

Merged a bunch of small changes from Docking branch to reduce the difference between branches. ...

Noticeable: horizontal alignment of CloseButton. Menu fill take account of border. Various stylistic tweaks to accomodate other changes in Docking.

Internals: Columns: Some renaming, extracted code into GetColumnsID().

Tests: Added hook/tweaks for imgui-test engine. + Fixed warnings.

Internals, Docs: Added a bunch of clarification about ButtonBehavior in the form of a table (and to facilitate writing tests)

Internals: Merge minor things from range_select branch. Added ImGuiButtonFlags_NoHoveredOnNav. Added IsItemToggledSelected() - unused here. Renaming.

Internals: Selectable: Added ImGuiSelectableFlags_AllowItemOverlap in imgui_internal.h (~ #684, #2341)

InputText: Made Shift+Tab consistently do nothing regardless of whether the back-end emits both char and keys or just keys. (#2467, #1336)

Internals: Renamed the ImGuiWindow HiddenFrameXXX fields to decorrelate them from resizing behavior, as those values are set by other logic.

Comments, todo list, remove trailing spaces.

Version 1.70 WIP

Version 1.69, comments, typos