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

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

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)

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

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

Viewport: Misc comments following user feedbacks..

Merge branch 'viewport' into docking ...

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

Viewport: Fixed not clearing request flags in main viewport, which led some back-end (SDL) to break on resize as PlatformRequestResize would stay true forever and inhibit new sizes passed to AddUpdateViewport(). (#1542)

Merge branch 'viewport' into docking

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp

Window, Focus, Popup: Fixed an issue where closing a popup by clicking another window with the _NoMove flag would refocus the parent window of the popup instead of the newly clicked window.

Merge branch 'master' into docking ...

# Conflicts:
#	imgui.cpp

Internals: Popups: Renamed CurrentPopupStack to BeginPopupStack which is much less ambiguous.

Internals: Popup related comments. Renamed the misleading internal ClosePopup() function. Added bool* test to BeginPopupModal in demo.

Tests: Changed prototype of ImGuiTestEngineHook_ItemAdd to match functions called in same spot. Made ButtonBehavior submit fallback item info if ItemAdd() was not called (for resize grips, resize borders, scrollbar, columns, etc.)

Merge branch 'viewport' into docking ...

# Conflicts:
#	docs/TODO.txt
#	imgui.cpp
#	imgui.h
#	imgui_demo.cpp
#	imgui_draw.cpp
#	imgui_internal.h
#	imgui_widgets.cpp

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Added BETA api for Tab Bar/Tabs widgets. (#261, #351) (merged this feature from the from Docking branch so it can be used earlier as as standalone feature) ...

- Added BeginTabBar(), EndTabBar(), BeginTabItem(), EndTabItem(), SetTabItemClosed() API.
- Added ImGuiTabBarFlags flags for BeginTabBar().
- Added ImGuiTabItemFlags flags for BeginTabItem().
- Style: Added ImGuiCol_Tab, ImGuiCol_TabHovered, ImGuiCol_TabActive, ImGuiCol_TabUnfocused, ImGuiCol_TabUnfocusedActive colors.
- Demo: Added Layout->Tabs demo code.
- Demo: Added "Documents" example app showcasing possible use for tabs.

Using named flags instead of 0 + shallow formatting tweaks from other branches.

Merge branch 'viewport' into docking + moved io.ConfigFlags ImGuiConfigFlags_DockingNoSplit to io.ConfigDockingNoSplit ...

# Conflicts:
#	imgui.cpp
#	imgui.h
#	imgui_demo.cpp

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui_demo.cpp

Tests: Adding imgui-test engine hooks (experimental) to provide missing widget state to the testing system.

Merge branch 'viewport' into docking ...

# Conflicts:
#	imgui.cpp

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Internals: Nav: Added ImGuiNavLayer_ to clarify semantic of previously integer NavLayer values, and not pretend that increment/decrement operators on them super flexible. + Storage tweaks.

Merge branch 'viewport' into docking ...

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

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp

Docking: Another attempt at tidying the inconsistent focus / tab bar reordering issues, and toward generalizing single window dock nodes. (#2109)

Viewport: Minor tweaks, better struct layout.