ImGuiTextBuffer: Avoid heap allocation when empty.

Comments + internal using Tab Stop terminology (ImGuiItemFlags_NoTabStop instead of !ImGuiItemFlags_AllowKeyboardFocus)

Viewport: Added ImGuiConfigFlags_ViewportsDecoration to re-enable platform decoration (#1542)

Viewport: Added ImGuiConfigFlags_ViewportsDecoration to re-enable platform decoration (#1542)

Docking: Kept ImGuiDockNodeFlags_PassthruDockspace and removed ImGuiDockNodeFlags_PassthruInEmptyNodes, ImGuiDockNodeFlags_RenderWindowBg. Doesn't include ImGuiDockNodeFlags_NoDockingInCentralNode. (#2109)

BeginTabItem: Fixed missing parameter name in .h file messing up with cimgui generator. (#2115)

Docking: Added ImGuiDockNodeFlags_PassthruDockspace mode (subdivided in three flags: ImGuiDockNodeFlags_NoDockingInCentralNode, ImGuiDockNodeFlags_PassthruInEmptyNodes, ImGuiDockNodeFlags_RenderWindowBg). Added internal facility for register a rectangular hit-test hole in window. Updated DockSpace demo accordingly. (#2109)

Docking: Added ImGuiDockNodeFlags_NoOuterBorder, tweaked DockSpace demo to remove window border. Made docking splitter use the same standard setting as resizing from edges. (#2109)

Docking: Renamed "DocRoot/DocumentRoot" to "CentralNode", more self explanatory. Moved Splitter update higher up in DockNodeUpdate() + minor misc tweak. (#2109)

Docking: Added ImGuiDockNodeFlags_NoDockingInsideDocRootNode flag. Honoring ImGuiDockNodeFlags_NoSplit in child node is already split (so we can use DockBuilder and then lock the layout). Added those options to the demo. (#2109)

Docking: Fixed DockSpace() child window displaying a scrollbar behind the node backgrounds. Rename ImGuiCol_DockingBg to ImGuiCol_DockingEmptyBg. Added ImGuiDockNode::IsLeaftNode(). (#2109)

Merge branch 'viewport' into docking

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp

Docking: Renamed io.ConfigDockingWithKeyMode to io.ConfigDockingWithShift.

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

Docking: Fixes for C++03 compilers.

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

Style: Added style.TabRounding setting.

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

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: 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

Docking: Renamed SetNextWindowDock() to SetNextWindowDockId(). Added GetWindowDockId().

Docking: Reworked SetNextWindowUserType() as SetNextWindowDockFamily(), allowing multiple scene to be tagged with their scene id and not receive tabs from other scene + persistent storage for family id to allow for docking tree fork/remap.

ImVector: Fixed a oddly unqualified return type in the assignment operator (I assume C++ handles it nicely as this never warned anywhere, but it is completely unintentional).

Docking: Renamed ImGuiDockSpaceFlags to ImGuiDockNodeFlags. Clarified in comments/demos that DockSpace creates a Node. Renamed IsExplicitRoot to IsDockSpace. Assert against explicitly calling DockSpace twice in a frame.

Fixed bad merge (a82be53) where we lost the version number in Docking branch + moved Docking related API together for now.