Organize src/ and test/ headers

Organize src/ and test/ headers

- Add context.h and move context-related functions from xkb-priv.h to
  it.
- Move xkb_context definition back to context.c.
- Add keysym.h and move keysym upper/lower/keypad from xkb-priv.h to it.
- Rename xkb-priv.h to map.h since it only contains keymap-related
  definitions and declarations now.
- Remove unnecessary includes and some and some other small cleanups.

Signed-off-by: Ran Benita <ran234@gmail.com>