Added sanity check to debug parent/child ordering issues (they would generally manifest with an assert/crash in EndFrame bu tthis assert will catch some earlier).

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)

Natvis: Added Hidden info about ImGuiWindow.

Viewport: Added minimum viable information in the Changelog.

Examples: Downgrading projects to xcode 9.2 (maybe 8.0) (#2134) ...

* example_apple_opengl2: The deployment target was set to 10.12 from XCode 9.2.
* imgui_impl_metal: header not found by XCode 9.2.
* example_apple_metal: The deployment target was set to 10.12 from XCode 9.2.

Moving issue/pr template to docs/. Added links in README.

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: Win32: Workaround to the fact that ::WindowFromPoint() seems to return Windows using ImGuiViewportFlags_NoInputs / HTTRANSPARENT when dragging nearby the platform title bar. This is to allow using platform decoration. I don't understand this well atm. (#1542)

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)

Examples: Comments about GLFW/SDL versions

Moved guidelines to issue #2261 to Pin and increase visibility for now.

Renamed extra_flags to flags in InputXXX parameters.