app: highlight the angles of GimpDial that are currently moved
app: fix off-by-0.5 drawing in GimpDial caused by ROUND() and a wrong offset
Doc that gimp-display-new gives execution error if no GIMP GUI.
app: clean up angle calculations in GimpCicrle and GimpDial
app: add private function gimp_dial_set_target()

which avoids invalidating the widget if the target didn't change.
app: factor gimp_prop_widget_new() out of gimp_prop_table_new()

The new function generically returns a widget for one property and is
used in gimp_prop_table_new().
app: rename gimpproptable.[ch] to gimppropgui.[ch]
app: remove the table from gimp_prop_gui_new()

and pack everything into boxes. There are a few size groups missing
to make everything look nicely aligned again.
app: restore random seed GUI for GEGL ops, broke that with the refactoring

and add a size group for the labels.
app: add gimppropgui-constructors.[ch]

for operation-specific GUI constructors. Factor a generic fallback
constructor out of gimp_prop_gui_new() and move it there.
app: add a little hint about reporting bugs against unstable git
app: experimentally add a custom GUI for gegl:convolution-matrix
app: add gimp_prop_angle_range_dial_new()

which is by no means general and needs to be improved if ever widely
used.
app: add a bad custom GUI for gegl:color-rotate
app: improce custom gegl:color-rotate GUI
app: convolution-matrix' matrix was flipped
app: add gegl:convolution-matrix to Filters -> Generic
pdb: replace \1 \2 \3 by $1 $2 $2 as suggested by perl warning
plug-ins, pdb: remove convolution-matrix and add a PDB compat procedure
app: add gegl:color-rotate to Colors -> Map