Viewport: Merge fix conflicting flags.

Examples, Platform, Viewport: Fixed inconsistent window ownership issues. Added comments. Made Win32/SDL back-ends track ownership.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx10.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/vulkan_example/main.cpp
#	imgui.h

Examples: Added missing calls for ::DestroyWindow, glfwDestroyWindow() + fix old comments.

Version 1.61 WIP

Version 1.60 (missed the string). Will retag.

Fixed erroneous call to io.Fonts->ClearInputData() + ClearTexData() that was left in DX10 example but removed in 1.47 (Nov 2015) in every other back-ends. (fixes 6cee2fc) (#1733, ~#1731) + assert

Nav: Fixed comment. (#1599)

Version 1.60

Internals: Removed unused internal variant of ArrowButton().

Fixed a few zealous warnings.

Fixed unused variable warning.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_dx9.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/vulkan_example/imgui_impl_glfw_vulkan.cpp
#	imgui.cpp
#	imgui.h
#	imgui_internal.h

Style: Default style is now StyleColorsDark()! Toward a 1.60 release :) (#707)

IO: ImGuiConfigFlags_NoMouse disable hovering so it is not destructive. Amend 2464e62

NewFrame: Extract some code into a NewFrameUpdateHoveredWindowAndCaptureFlags() which can be of use with touch-based inputs systems.

Comments about io.WantCapture flags + todo entries.

NewFrame: Extract some code into a NewFrameUpdateMouseInputs function. Moved settings saving higher up in the function so it is next to its peers.

IO: Added ImGuiConfigFlags_NoMouse to force clear/disable mouse inputs in NewFrame().

Renamed ImGuiConfigFlags_NoSetMouseCursor to ImGuiConfigFlags_NoMouseCursorChange. Followup to 75c3793 two weeks ago. (#787, #1495, #1202) + comments

Columns: Fixed calling Columns() again with the same number of columns. (#125)

Viewport, Platform, Examples: Added support for transparent window via PlatformIO Platform_SetWindowAlpha (#1542) + fixes for GLFW 3.3

Viewport: Comments about honoring ImGuiViewportFlags_NoInputs and MouseHoveredViewport. (#1542)

Viewport: Moving a window accross viewports tries to preserve the dragging pivot (that is assuming that the window will be evenly scaled by DPI, aka Style is scaled). (#1542)

Viewport: Renamed RenderPlatformWindows() to RenderPlatformWindowsDefault(), tweaked examples and emphasis optional Viewport code path. (#1542)

Contribution guidelines

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_dx9.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.h
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.h
#	imgui.cpp

Hide new windows for one frame until they calculate their size. Also fixes SetNextWindowPos() given a non-zero pivot. (#1694)

Internal: using more consistent comparaison with HiddenFrames to not imply it could be -1. Tweaked comments.

Added extra comments and assertions to avoid user combining ImGuiCond flags. (#1694)