Merge branch 'viewport' into docking

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp

Window, Inputs: Fixed resizing from edges when io.MousePos is not pixel-rounded by rounding mouse position input. (#2110)

Window: Resizing from edges (with io.ConfigResizeWindowsFromEdges Beta flag) extends the hit region of root floating windows outside the window, making it easier to resize windows. Resize grips are also extended accordingly so there are no discontinuity when hovering between borders and corners. (#1495, #822, #2110)

Docking: Fixed crash using DockBuilderSplitNode(). (#2109)

Docking: Renamed io.ConfigDockingWithKeyMode to io.ConfigDockingWithShift.

Fixed typo. (#2108, #2083)

Drag and Drop: Added GetDragDropPayload() to peek directly into the payload (if any) from anywhere. (#143)

Docking: Better tracking of current dock id for inactive and uncreated windows (in settings etc.). + Fixed assert when docking a single-visible leaf node of a hierarchy into another

Docking: Fixes for C++03 compilers.

Docking: Fixed extracting a dock node from a document root: document root should be preserved.

Docking: Reorganizing some of the tab-bar selection and window focus related code.

Docking: Demo: Displaying a message if master docking flag is disabled. + DockSpace() early out + comments.

Docking: Various sanity fixes + fixed tab-bar items sorting issue when docking single window with a dock node into single window without dock node.

Docking: Fix DockContextRebuild() temporarily invalidating dockid reference leading to undesirable merging of nodes.

Docking: Fixed dock node with single window accidental stealing of non-owned host viewport (leading to general havoc).

Style: Added style.TabRounding setting.

Docking: Added DockBuilderCopyDockspace() wip. Added DockBuilderCopyWindowSettings(), renamed DockBuilderForkNode() to DockBuilderCopyNode().

Fixed extraneous static attribute. (#2105)

Fix missing IMGUI_API and mismatching namespace internal's CreateNewWindowSettings (#2105)

Added FindOrCreateWindowSettings() internal helper.

Docking: Reworked DockContextPruneUnusedSettingsNodes() to prune entire unused trees.

ImPool: Added Reserve() helper function.

Merge branch 'viewport' into docking

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_win32.cpp
#	examples/imgui_impl_win32.h
#	imgui.cpp

Renamed SetScrollHere() to SetScrollHereY(). Kept redirection function (will obsolete).

BeginChild(): Fixed BeginChild(const char*, ...) variation erroneously not applying the ID stack to the provided string to uniquely identify the child window. This was undoing an intentional change introduced in 1.50 and broken in 1.60. (#1698, #894, #713) + reworked the Begin/BeginChild comments in imgui.h.

Docking: Fixed tab order restoring (follow up to 67be485e, broken in 455dc6e2)

Docking: Added DockBuilderForkNode() primitive for usage for user code to fork/copy entire layouts. This will probably need to be added to as I find ways to migrate patterns from user code to the library. Added IMGUI_DEBUG_DOCKING_INI helper to facilitate debugging. (+1 squashed commits)

Docking: Minor renaming. DockContextAddNode() uses 0 as marker for automatic ID selection + TODO update and moved docking entries to docs/TODO.txt