Cleanup: Rename: Static Override -> Library Override.

Better to make internal code naming match official/UI naming to some
extent, this will reduce confusion in the future.

This is 'breaking' scripts and files that would use that feature, but
since it is not yet officially supported nor exposed in 2.80, as far
as that release is concerned, it is effectively
a 'no functional changes' commit.

Fix T65109: Object deleted when removed from the RigidBodyWorld collection.

While user should never do that, it appears many end up using a 'view
layer' instancing collection as RBW collection, and even worse, have
objects in that unique collection.

Therefore, when removing RB simulation from an object, which among other
things has to remove it from the RBW collection, it would fully delete
the object from the blend file.

This fix merely checks the usercount of RB-removed object, and if it is
at 1 (which means object was in a single collection), it adds it to the
scene's master collection first.

Clear Datablocks Preview: add some more generic options.

Sometimes one may want to nuke all previews altogether, running the
operator for all eight individual ID types would be tedious then...

Hide from UI overridable option for custom properties.

Fix/Cleanup: I18N: Bad usage of IFACE_ instead of TIP_.

Cheap tip: anything that is not "Camel Case" and/or that is more than
a few words long should use `TIP_` translation, not `IFACE_` one.

Also added several missing strings (including the one reported in D5056
by Jean First (@robbott), thanks).

Cleanup: remove unused var.

Fix T64430: Removing objects from sub-collection doesn't update instances of parent collection.

Collections are a tad annoying with all their caching of objects... When
we modify content of a children collection, we need to tag DeG for CoW
update of all of the ancestors.

For now keeping that recursive tagging helper private, but would not be
surprised if we found more similar cases and needed to expose it to more
code...

Fix (unreported) message wrongly using IFACE_ for its translation.

`IFACE_` is for short strings always shown in UI (like labels of buttons,
menu entries...). Every thing else, especially when more than a couple
of words, must use `TIP_`.

Fix T64608: DOF Focus object, not linked to new copy, Scen "full_copy".

Note that there are probably many other similar cases... This code is
really legacy, should use library_query helpers and other modern
BKE_library code instead of doing its own dirty cooking...

Fix T65295: Convert curve to mesh fails with Keep Original.

This code now expects to wrok from fully evaluated data, however when we
keep original, we are actually working from data just copied from orig
one.

Ideally, we'd do a single depsgraph update/eval *after* we have created
all new required data, but that is tricky to do properly in that code
without risking breaking one thing or another.

So for now, just going for the simple, if not optimal solution, and just
repeatedly re-evaluating whole deg every time we duplicate an object to
be converted. Yep, dummy, but simple and... safe. ;)