Bug 784802 - Crop and rectangle-select tools incorrectly detect ...

... current aspect ratio

When updating the default aspect ratio of a widget-less crop tool,
construct a temporary GimpToolRectangle widget, so that we can use
it to call gimp_tool_rectangle_constraint_size_set() and pick the
correct ratio, instead of just bailing.

When halting the crop tool, update the default aspect ratio, which
now does the right thing, as per the above.

Update the default aspect ratio upon changes to the active layer of
the current image, and to the size of the active layer, which
affect the default aspect ratio when "current layer only" is
toggled.