Bug 605872 - Units dropdown list is not updated

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.

(cherry picked from commit 66298ba869dac811605e0a93e804cbac257cbf73)