Rewrite the ConstraintVisualizer yet again.
The interactive parts become an Interactor, and the visual changes in the last iteration are mostly reverted. The handles are simple rect items again, and now they are always visible to improve the discoverability of this feature. This commit also solves a long-standing bug: Sometimes, when enlarging the scene rect beyond the view's boundaries, its size increased much more than wanted. This is now gone. svn path=/trunk/KDE/kdegames/palapeli/; revision=1111398
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