You can now set any paint tool to mirror painting relatively
horizontal/vertical axis or a central point (any combination of these 3
symmetries).
This has been implemented as a new multi-stroke core, where every stroke
is actually handled as a multi-stroke (default of size 1).
This is also the first usage of custom guides for symmetry guiding.
Current version has to be activated in the playground.
This adds a boolean to GimpCoords struct that is true for enabled
extended non-mouse devices and false for all the rest allowing
the mypaint brush to override the the pressure sent to the paint library.
