`gimp_unit_store_get_value()` clashes with the `get_value()` method of
its parent class GtkTreeStore. This means trouble for bindings, as seen
here in some pseudocode:
```vala
var unit_store = new Gimp.UnitStore();
// Which function are we referencing here, the one from GtkTreeStore or
// the one from GimpUnitStore? Worse, they both have different arguments
unit_store.get_value( ... )
```
even if we don't have private members (yet). Also make class padding 8
pointers in all headers. This commit moves nothing to private, it just
makes all headers consistent and adjusts .c files accordigly.
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.
This only helps to maintain proper includes in app/ and shouldn't
affect plug-ins at all, because these are supposed to only include the
main headers from libgimp/ since the beginning of time.
The gimpfootypes.h files do not have these guards, so we can continue
to maintain app/'s include policy that is very likely to error out if
wrong things are included.
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.
Niels De Graef