Docking: Fixed losing tab bar selection when extracting a whole docked node + reusing existing dock node when possible.

Merge branch 'viewport' into docking ...

# Conflicts:
#	examples/example_glfw_opengl2/main.cpp
#	examples/example_glfw_opengl3/main.cpp
#	examples/example_glfw_vulkan/main.cpp
#	examples/example_sdl_opengl2/main.cpp
#	examples/example_sdl_opengl3/main.cpp
#	examples/example_sdl_vulkan/main.cpp
#	examples/example_win32_directx10/main.cpp
#	examples/example_win32_directx11/main.cpp
#	examples/example_win32_directx12/main.cpp
#	examples/example_win32_directx9/main.cpp

Docking: Fixed undocking on whole dock node from leaving undesirable empty node in the docking tree. Fixed calls to MarkIniSettingsDirty().

Nav: Use Platform_SetWindowFocus when CTRL+Tabbing to another viewport.

TabBar: Recover if SelectedTabId doesn't exist anymore.

ImDrawList: Fixed AddConvexPolyFilled() undefined behavior when passing points_count smaller than 3, in particular, points_count==0 could lead to a memory stomp if the draw list was previously empty.

Viewport: GLFW: Build fix for pre GLFW 3.2 (#1542)

Examples: Referring to missing gamepad support in back-end that are missing it.

Viewport: Enabled viewports by default in more example apps.

ArrowButton(): Fixed arrow shape being horizontally misaligned by (FramePadding.y-FramePadding.x) if they are different. ...

Demo: Added extra usage of AlignTextToFramePadding() in a more visible section.

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.

Docking: Added undocking of whole dock node by dragging from the Collapse button. Super useful and works great!

Docking: Fix of title bar flicker during rebuild. Fixed dragging of DockNode always triggering BeginAsDockableDragDropSource() when io.ConfigDockingWithKeyMod is false.

Docking: Internals renaming LastExplicitSize to SizeRef (invalidate .ini data) + todo

Docking: Creating tab bar and adding window into tab bar immediately (not sure why I didn't do that before). Probably broke something subtle, will find out. *Edit* Initially broke restoring selected tab.

Docking: work to allow programmatic control of dock nodes, various refactor + assert fix. Probably broke something (but I haven't found what yet!)

Contributing, Issue Template

Docking: Internals: Transitioning some code toward consistently using ImGuiContext* ctx parameter instead of ImGuiDockContext

Docking: Fixed central docking display with ImGuiDockSpaceFlags_NoSplit, affecting in particular with io.ConfigDockingWithKeyMod = false. + comments, asserts

Added Fonts readme details and links to Sweet16 font. (#2085)

Fix file names (#2083)

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

Docking: Changed DockSpace() signature to take an ID, as ID will frequently be used by other API it makes more sense to let the user compute it.

Docking: Removed SplitRatio from ImGuiDockNode and ImGuiDockNodeSettings, which we don't need anymore.

Docking: Misc rework/rename toward being able to rebuild a branch selectively, so we can honor settings changes on a per Dockspace basis. + Comments

Merge branch 'viewport' into docking ...

# Conflicts:
#	docs/CHANGELOG.txt
#	imgui_demo.cpp
#	imgui_internal.h

Viewport: Increased threshold for setting ImGuiViewportFlags_NoFocusOnAppearing. (#1542), 2 doesn't seem enough with some docking setup.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx9.cpp
#	examples/imgui_impl_opengl2.cpp
#	examples/imgui_impl_opengl3.cpp
#	examples/imgui_impl_vulkan.cpp

BeginMenu(): Fixed menu popup horizontal offset being off the item in the menu bar when WindowPadding=0.0f.

ColorPicker: Replaced the Separator with a Spacing call. (#2068)