QmlDesigner: Refactor layout functions

Change-Id: If95aead7d922511e4f1b3abdee38f4c403da37a5
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
VCS: Un-data functions that accept QString

Change-Id: Iffa82f4ab06162ab57e77301e77fb37ed7230bd5
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Fix some memory leaks.

Change-Id: Iba9d32aacbfe11d9542f629fca0571998b4fadfb
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Use proper way of looking for system Qt

Change-Id: I770a15c653c6b25d10a919f6c6dbefdb79c17d89
Reviewed-by: Daniel Teske <daniel.teske@digia.com>