Many changes:

- Handle icons for Windows, Mac and Linux:
	Windows - untested but should work
	Mac			- needs .icns file
	Linux		- handle copying gdclient.png to /usr/share/pixmaps/gdclient.png, fallback to freedesktop application-internet icon

- Handle users changing nicknames
- Do not create a new list of users for autocompletion but rather use _userList
- Remove some superflous includes and forward declarations of classes