This commit adds support for moving together an intersecting pair
of guides using the Move tool, by dragging the guides at their
point of intersection.  This is useful when the guides are used to
mark a point, rather than a pair of lines (e.g., as is the case for
the mandala symmetry guides, which mark the symmetry's point of
origin).

Add gimp_image_pick_guides(), which can return a set of guides,
rather than a single guide.  The API allows an arbitrary set of
guides to be returned, but, in practice, at most two intersecting
guides are returned, as per the above.

In GimpMoveTool and GimpGuideTool, add support for moving multiple
guides together, and, in GimpMoveTool, use gimp_image_pick_guides()
to potentially pick multiple guides.