Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_sdl.cpp

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..

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

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)

Made it illegal to call Begin("") with an empty string. This somehow accidentally worked before but had various undesirable side-effect as the window would have ID zero. In particular it is causing problems in viewport/docking branches.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui.h

Viewport: Corrected/clarified comments. Moved RenderPlatformWindowsDefault() next to UpdatePlatformWindow(). Removed unnecessary flag check.

IO: Added BackendPlatformUserData, BackendRendererUserData, BackendLanguageUserData void* for storage use by back-ends. (#2004 + for cimgui)

Fixed using SetNextWindowPos() on a child window (which wasn't really documented) position the cursor as expected in the parent window, so there is no mismatch between the layout in parent and the position of the child window. Demo tweak and adding some child window stuff

Various user-facing comments

Tidying up README, moved entries to FAQ, updated screenshots, removed comma in title.

Fix merge issue on master.

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.

Moved setting up NavHideHighlightOneFrame from lower-level ClosePopupToLevel() to CloseCurrentPopup() with an explanation. (Followup on 68d3e13)

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.

Viewport, DPI: Now using DpiScale from the ImGuiPlatformMonitor array instead of calling Platform_GetWindowDpiScale() before the platform window creation. Might even tentatively see if things work out without Platform_GetWindowDpiScale. (#1676)

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 'master' into viewport ...

# Conflicts:
#	imgui.h

Comments, minor tweaks.

Metrics: Fixed crash when viewports are disabled (g.MouseLastHoveredViewport is never set).

Internals: Windows hidden with HiddenFramesRegular (but NOT HiddenFramesForResize) preserve their SizeContents, so restoring a auto-resize window after it's been hidden by tabs won't reset its size for a frame. Arguable. Let's see how it goes. (Followup to b48e295)

Contents size is preserved while a window collapsed. Fix auto-resizing window losing their size for one frame when uncollapsed.

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.