Add gimp_prop_unit_combo_box_new() and adapt all places using the
prop_unit_menu. Some things are broken now, like there are no pixel
digits set, resolution unit menus show "pixels" and warn badly when
pixels is selected, and file-pdf-load is not built right now.
More fixes to come...
Created a resource file for plug-ins and a rule to link it to them.
The application icon file is smaller than the default one, there's no
point in doubling the size of a plug-in executable just for an icon.
If no problems turn up, this will close the bug.