Minor keyboard input fixes

[samhocevar]

This PR tries to improve consistency across desktop platforms:

• autorepeated keys on Windows (already works on Linux and OS X).
• backspace triggers kinc_internal_keyboard_trigger_key_press() on Windows.
• backspace, return, and tab trigger kinc_internal_keyboard_trigger_key_press() on OS X.

[RblSb]

Isn't autorepeating should be disabled on all platforms and implemented by user? At least this is basic behavior for keydown, not sure if this change about keypress and what behavior for keypress is preferable

[samhocevar]

The application can just ignore repeated keys if it wants to implement its own autorepeat mechanism, so nothing is lost.

Also, exposing repeated keys ensures that the autorepeat delay and rate are exactly what the user configured in their operating system. I think that’s a good thing.