Bug 865033 part 1. Remove GetLayerManager from CompositorParent. r=mattwoodrow
Bug 865033 part 2. Add mCompositor to CompositorParent. r=mattwoodrow
Bug 946958 part 1. Remove GetWidgetSize. r=mattwoodrow
Bug 946958 part 2. Remove methods which just forward to compositor. r=mattwoodrow
Bug 946958 part 3. Remove thebes callbacks. r=mattwoodrow
Bug 946958 part 4. Tidy up interaction with widgets. r=kats
Bug 946958 part 5. Misc tidy ups. r=mattwoodrow