Merge branch 'viewport' into docking ...

# Conflicts:
#	docs/CHANGELOG.txt
#	imgui.cpp
#	imgui_demo.cpp

(Breaking change) Reorganized Viewports advanced flags, moved into new io.ConfigViewportsXXX flags. Pay attention that ImGuiConfigFlags_ViewportsDecoration became ConfigViewportsNoDecoeration, so the value is inverted! (#1542)

Viewport: Added ConfigViewportsNoParent to parent viewport default to NULL and not main viewport. Fix eg.. popups appearing erroneously focusing parent window.

Viewport: Fix for minimization of individual viewports (the current back-end forcing a parent/child relationship between secondary viewports and the main viewport have hidden this issue). Follows d8ab2c1ac.

Viewport: Fixed minimization of main viewport leading to it being omitted from platform_io.Viewport list where the users assume it is at index 0. Fix d8ab2c1ac. It wasn't a problem when other viewports were child of the main viewport because they would all be minimized together. (#1542)

Viewport: Added minimum viable information in the Changelog.

Docking: Merge fix duplicate line + added assert to ease debugging.

Merge branch 'viewport' into docking ...

# Conflicts:
#	imgui.cpp

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Internals: Extracted code out of EndFrame() into UpdateMouseMovingWindowEndFrame()

Alternative fix for bug introduced in d845135 (#1651), fix CTRL+Tab and fallback tooltip.

Revert "Fixes crash/assert bug introduced in d845135 (#1651): would assert when showing the CTRL+Tab list and or fallback "...." tooltip." ...

This reverts commit 1b0e38d.

Fixes crash/assert bug introduced in d845135 (#1651): would assert when showing the CTRL+Tab list and or fallback "...." tooltip.

Docking: Forward WindowClass from node to host window.

Merge branch 'viewport' into docking ...

# Conflicts:
#	imgui.cpp
#	imgui.h
#	imgui_internal.h

Viewport: Added ImGuiWindowClass / SetNextWindowClass() (concept imported from Docking ImGuiDockFamily), which currently allows to overwrite viewport flags on a per-window basis. Exposed FindViewportByID(). Win32: Support for ParentViewportId. (#1542)

Docking: Renamed SetNextWindowId() -> SetNextWindowID() for consistency. (function vs member are still horribly inconsistent atm)

Viewport: Added Platform_UpdateWindow hook for general purpose: Rework Win32 code to reflect viewport flags changes into Win32 while the window is active.

Viewport: Reorder flags. Set owned viewport common decoration flags in Begin(). Moved code in UpdateViewportsEndFrame() before we introduce family/class based overrides.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_sdl.cpp

Docking: Agressively assert when CentralNode is a not a leaf node in order to find our bug.

Renamed ImGuiDockFamily to ImGuiWindowClass. Renamed CompatibleWithClassZero to DockingAllowUnclassed. (#2109)

Happy new year! & comments

Comments: fixed missing line in the "how a simple rendering function may look like" section (#2258)

Error recovery: Extraneous/undesired calls to End() are now being caught by an assert in the End() function itself at the call site (instead of being reported in EndFrame). Past the assert, they don't lead to crashes any more. Missing calls to End(), pass the assert, should not lead to crashes any more, nor to the fallback/debug window appearing on screen. (#1651).

Tweaked asserts

Viewport: Misc comments following user feedbacks..

Fix merge in Docking branch, remove ConfigDockingWithShift flag from DX11 example + misnamed function.

Merge branch 'viewport' into docking ...

# Conflicts:
#	examples/example_allegro5/main.cpp
#	examples/example_win32_directx9/main.cpp

Merge branch 'master' into viewport + added style tweak block when viewports are enabled ...

# Conflicts:
#	examples/example_glfw_opengl3/main.cpp
#	examples/example_sdl_opengl3/main.cpp
#	examples/example_win32_directx11/main.cpp