Regression in 427eed292d.
Root of the issue was that animation system was using a single same
check to decide if an F-Curve/driver was valid to use to animate some
data, and whether user can create/edit animation for that data.
Both cases are actually different, since e.g. linked data is not
user-editable, but it can still be animated (either by related linked
Actions, drivers defined in the linked data, or some more hackish
changes like py API/RNA scripting).
This commit now defines two checks:
* `RNA_property_animateable`: whether a RNA pointer & propoerty is
animateable, based on their types and definition.
* `RNA_property_anim_editable`: whether a specific data referenced by
the RNA pointer and property is effectively user-editable.
The new `driveable` check added by 427eed292d is also renamed to
`RNA_property_driver_editable` (since the basic type-based
`RNA_property_animateable` is also valid for drivers currently).
Bastien Montagne