Merge branch 'master' into viewport (inc Vulkan resize fix merge #1042)

Internals: Columns renaming fields

Viewport, DPI: Some early work on per-viewport DPI support. At the moment the easiest way is to replace fonts during the ChangedViewport callback, but down the line we should aim at handling some of it at ImFont level. (#1542, #1676)

Viewport, DPI: Select viewport before locking style sizes and before handling double-click-on-title-bar to collapse. (#1542, #1676)

Merge branch 'master' into viewport

Viewport,Platform: Added GetWindowDpiScale() platform interface, changes of scale are reflected by positioning and resizing windows in their given viewport. (#1542)

Viewport: Coupling non-main viewport to one window. Viewport name derived from the window. Added ImGuiConfigFlags_PlatformNoTaskBar, off by default (aka re-established task-bars by default for now). (#1542)

Drag and Drop: Increased payload data type to 32 characters. (#143)

Viewport: Disable extra viewports from hosting other windows. (#1542)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_glfw.h
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.h
#	imgui.cpp

Viewport: Tracking current viewport we are appending to + added callback in PlatformInterface for DPI purpose (WIP). (#1542)

Columns: Extent stop at the right-most clipped pixel. The right-most column might appear a little wider but it's usable space matches the others. (#125). +9 Internal: Store InnerClipRect.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/allegro5_example/main.cpp
#	examples/directx10_example/main.cpp
#	examples/directx11_example/main.cpp
#	examples/directx12_example/main.cpp
#	examples/directx9_example/main.cpp
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_dx9.cpp
#	examples/imgui_impl_glfw.h
#	examples/imgui_impl_vulkan.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl2_example/main.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp
#	imgui.cpp
#	imgui.h

Viewport: Give a chance to platform and renderer to actually destroy their data. Otherwise the regular ImplXXX_Shutdown + following by DestroyContext() order fails to fullifl this. (#1542)

Renamed ImGuiStyleVar_Count_ to ImGuiStyleVar_COUNT for consistency with other enums. Added

Added ArrowButton(). Exposed ImGuiDir.

Internal: renamed RenderTriangle() to RenderArrow().

Internal: Moved IM_NEWLINE helper to imgui_internal.h

Renamed ImGuiNavFlags io.NavFlags to ImGuiConfigFlags io.ConfigFlags. (#787)

Renamed ImGuiNavFlags io.NavFlags to ImGuiConfigFlags io.ConfigFlags. (#787)

Viewport: Initial viewport branch+ platform api commit (squashed from previous commits, still very WIP) (#1542)

Clearing next window data at the end of Begin() more reliably. Misc comments.

Internals: Settings: Added ReadCloseFn to allow handlers to sanitize data on a per-entry basis.

ImRect: Removed misleading IsFinite() function used by some Nav code.

TreeNode: Renamed Beta ImGuiTreeNodeFlags_NavCloseFromChild to ImGuiTreeNodeFlags_NavLeftJumpsBackHere. (#1079)

Nav: Internals: Exposed NavMoveRequestCancel in imgui_internal.h (#1640)

Initialization happens during CreateContext(), which makes it easier for platform layers to interact with the context during their initialization.

Internals: Clarified internal storage for RootWindow with specific semantic (the Docking branch will need to introduce finer differenciation there.)

Updated version to 1.60 WIP to emphasis on the addition of required calls CreateContext/DestroyContext + merged the massive Navigation branch (#1565, #787)

Merge branch 'context' ...

# Conflicts:
#	examples/allegro5_example/main.cpp
#	examples/directx10_example/main.cpp
#	examples/directx11_example/main.cpp
#	examples/directx9_example/main.cpp
#	examples/marmalade_example/main.cpp
#	examples/opengl2_example/main.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp