Enables aligning of elements using the active Path

For some reason, using the image active Path as
a reference for the align tool was displayed
in the UI, but disabled in the code.
Adds "fill" arrange modes to the Align Tool

The Align Tool had to be used in a very hacky way if one
intended to evenly distribute items across an image,
or other reference object (it would actually require one to
calculate the item spacing out of GIMP). This adds vertical
and horizontal fill modes: the reference object is divided
in N equal segments, where N is the number of items,
and each item is placed in the center of one of
these segments. The existing "offset" parameter
is used as an extra margin for the distribution,
and can be set to negative values, so that the items can
even get moved outside the boundaries of the reference object.