Examples: Most examples apps detect if ImDrawIdx was changed to 32-bits instead of 16

Examples: Possibly clarified the intent of imgui_impl_* files.

Examples: comment on setting the RenderDrawListsFn function.

Merge remote-tracking branch 'origin' into 2015-03-antialiased-primitives ...

Conflicts:
	imgui.cpp

Allegro 5 example: removed public domain mark, MIT as the rest, with @bggd approval

Allegro 5 example: converted for indexed rendering.

Allegro 5 example: fixed uv coordinate of non-default texture.

Allegro 5 example: handling of hardware and software mouse cursor rendering.

Allegro 5 example: fixed key modifiers, keyboard input, clipping, added pagedown/pageup.

Allegro 5 example: formatting, match other example structure, fixed mouse buttons.

Allegro 5 example: main.cpp matches other examples. added window title.

Allegro5 example: added example from https://github.com/bggd/a5imgui_example