Qt4projectManager: Refactor Qt4Target, split up in per target classes

Almost(*) all of the symbian and maemo specific code is now theoretically
moveable to a separate plugin. Thus making it possible to implement
new targets in a plugin.

(*) Noteable missing is the qtversion, which needs to be split up
per target too.

Also fixes
Task-Nr: QTCREATORBUG-2440

Reviewed-By: hunger
Reviewed-By: ck