RNA collections that support insertion of new items in liboverride
data-block need a special way to distiguish between locale and
orig-from-linked items (since some operations are allowed on the forer,
but no the latter).
In future we want a proper solution to abstract that at the
`BKE_lib_override` level, but for now we need to add some code for each
case.
Note that this commit also fixes a few potential issues with GPencil
modifiers, and constraints, regarding their handling of local overrides.
bab57550b6