* Fix strange behavior on pressing "Kana" and "Eisu" keys.
  -> One of cause of this issue is GDK's keymap. Currently the key values of
     them in GDK world are same with "Space" key.
* Fix the issue that preedit string isn't displayed on Xamarin Studio.
  -> It seems Xamarin Studio's issue. When I set foreground & background
     attributes to preedit string, it shows the preedit string correctly.
     It should show it without these attributes.
* Fix cursor location (especially on multi display environment).
* Sometimes the position of a double underline for a preedit string seems wrong
  (unconfirmed).
* Implement "reset" feature.
* Implement "focus_in" and "focus_out".
* Implement "set_use_preedit".
* Hook toplevel window's key event.
