Optimize key events

Extend KeyPressed/KeyReleased events:
* add more keys
* include native key code in KeyEventArgs

MAC: add KeyPressed/KeyReleased and TextInput implementation
* key mapping from NSEvent.h
* NSEvent.KeyCode contains US codes, so I use the Characters provided by NSEvent for translation of writable keys. (NSKey from MonoMac does not match many keys, have I missed some conversion?)