Examples: Possibly clarified the intent of imgui_impl_* files.

Examples: imgui_impl* functions declared with IMGUI_API to ease copy & pasting into projects using DLL

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

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