app: GimpToolRectangle multi-layer aware.
This fixes default constraint values when multiple layers are selected. In particular fixed aspect ratio defaults in Crop or Rectangle Select tools, in layer mode, when several layers are selected, the default aspect ratio is the layer's ratio if all layers have the same dimensions, otherwise it's the image dimension ratio. Also fixes a bug when trying to create the tool rectangle while multiple layers are selected (the rectangle was always of size 0 at point 0). Finally fixes the auto-shrink feature with multiple layers selected. The rectangle will now auto-shrink to the smallest rectangle which encompasses the contents of all selected drawables inside the existing rectangle.