QmlJS check: Extend "don't use var" performance warning.

* now also recognizes color, rect, point, size, vector3d
* to do detect these correctly, set the return types on a number of
  builtin Qt.* functions
* add test

Change-Id: Ieaeb73be208af2d47e1bb4fa1485dc876705ee20
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>