Composer rotation fixes and improvements

This branch goes a long way toward fixing rotation of composer items. I've made it possible to rotate all types of composer items (fixes #4884) and improved how QGIS handles rotated items (fixes #7933). Now, resizing of single rotated items or selections/groups of items with the same rotation are done using the same axis as the rotated items. This mostly fixes  #8466 and #8125, at least for the most common use cases. (Still remaining to do is fixing resizing of selections consisting of items with differing rotation, but this depends on shear/perspective transforms, which is still a wip and is a much rarer use case). Additionally, using new methods available in qt 4.6 helped clean up and simplify the old rotation code.
Still to come is rotation rescaling which depends on shear/perspective transforms, and a comprehensive test suite. In the meantime it should be safe to get this merged for further testing and feedback.