_gimp_unit_store_sync_units(): don't trigger a sync of newly added
units if we encounter GIMP_UNIT_PERCENT, it has a special value that
is always larger than any other unit. All unit menus containing
"percent" were broken the same way.
Add private API _gimp_unit_store_sync_units() which emits
"row-inserted" on each unit that didn't exist when the GimpUnitStore
was created, or when sync_units() was called the last time.
In GimpUnitComboBox, call sync_units() each time the combo is popped
up, or a unit is set on the combo.
to allow having unit combos without "pixels". Adapt GimpUnitComboBox
to not assume that the unit is equal to the index in the store and
enable "menu_has_pixels" in GimpSizeEntry again.
