Layer Select Contents action

The lack of this feature was driving me nuts so I dropped it in!

* Added a Menu Item in PXLayerController for "Select Contents" 
* Added selectContentsOfLayerAtIndex: method in PXCanvas_Layers.m that constructs a selection mask from the layer then sets it as the PXCanvas' selection mask.

I first tried to recycle some methods from PXCanvas_Selection, but that approach was much slower than manually iterating the tiles and constructing the selection mask.