...to GimpProcedureDialog.
This patch allows plugin developers to create a
GimpCoordinates widget (two GimpSizeEntries linked
together with a chain and with a units combo) in the
GimpProcedureDialog.

It is largely a wrapper for gimp_prop_coordinates_new ().
This patch also makes that function more responsive
to the unit property, now that GimpUnit has a proper
GParamSpec implementation. The default unit and
whether we show pixels, percents, or the resolution
spinners can now be determined via the unit
parameter (if set) rather than relying on
hardcoded values.

As a demonstration, the Tile plug-in's width/height
entries were restored to what they were before the
GimpProcedureDialog port, with units and a chain
link between values.