Adds more special keys and key parsing

[data-niklas]

ParseKey returns a Key from a string. E.g.: "a" -> hotkey.KeyA. The second returned value indicates, if the Key was found.
Also added special keys:

• Return
• Escape
• Delete
• Tab
  Keycodes taken from linked posts / files.

Tested ParseKey on Linux. It at compiles, runs and at least works for "a" and "Return". Also tested KeyReturn.

It would be great to parse whole hotkeys from e.g.: Control + a.

[changkun]

Thanks for the contribution, I left a comment regarding the keycode conversion.

[changkun]

Thanks!