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.
...when moving guides
Show relative coordinates when moving guides and sample points, the
cursor display in the statusbar already shows absolute coordinates.
Add two new tools, GimpGuideTool and GimpSamplePointTool. They are
one-trick-ponies and can only create new or move existing guides and
sample points. They can't be selected from the toolbox, only
temporarily pushed as active tools on top of any active tool using
their public start() APIs.
Use that API to enable them when the rulers are clicked, and replace
the entire guide and sample point moving code in GimpMoveTool and
GimpColorTool by simple calls to that API.
This might look like overkill but can easily be used for other
features like moving guides from within the paint tools (mirror
painting) or gegl filters (preview curtain).
Ell