Cleanup the code just a bit
Fix Alt for single keycode special keys

- We now also encode the Alt modifier in special keys with a single
  keycode (e.g. Return, Space, Tab)
- Removed the code that handled modifiers for Ascii sequences
  Qt is already handling Ctrl, and there is nothing usefull to
  be done about Shift
- Added Enter and Space to the special keys table - we
  can now use modifiers with these keys as well
Unify keyboard input handling functions

- This commit refactors most of the keyboard input handling code
  to handle all of it in a single method
- Removed the old specialKey() method, everything is now handled in
  the keyPressEvent() method
- Cleaned up the special key table