... and rename "clockwise" to "clockwise-delta"

Add a boolean "clockwise-angles" property to GimpDial, which, when
set, causes the dial legs' angles to be measured clockwise, rather
than counter-clockwise.  The property is FALSE by default.

Rename the "clockwise" property, which controls the direction of
the measured delta between the two angles, to "clockwise-delta", to
avoid confusion, and adapt the rest of the code.