Remove rigid body from rbw->group when deleting

I moved some code from ED_rigidbody_object_remove() to
BKE_rigidbody_remove_object(), so that calling the latter doesn't leave
the object in rbw->group (causing a crash later on when rebuilding the
depsgraph).

Fixed rigid body simulation randomly stopping

This was caused by a floating point comparison using ==, instead of using
compare_ff_relative().

RigidBodyWorld: copy ptcache from evaluated scene back to original scene

This makes it possible to perform re-evaluation of the scene without
having to re-run the simulation.

The CoW ? Orig copy is only performed when the depsgraph is active, so
as to not influence the current scene while rendering in the background.

Alternatively, we could have the CoW copy share the cache with the
original to prevent too much copying of cache data. This will be faster,
but I'm not sure whether we can reliably check the DEG_is_active()
status at CoW copy creation time.

Show cached frames in timeline at correct height

The line was covered by the horizontal scrollbar instead of drawn above
it.

Dependency graph fixes for RigidBodyWorld

- rbw->group added to the depsgraph.
- Mesh evaluation added when necessary.
- Prevent of double-free by freeing the scene before objects.

Converted Rigid Body from DerivedMesh to Mesh

Give RigidBodyWorld collection a fake user

This prevents it from disappearing when the blend file is saved.

Improved two tooltips

Tooltips shouldn't just repeat the information from the label.

Use evaluated objects in Alembic unit tests

This at least makes sure the tests don't fail any more. Possibly there
should be more evaluation happening there.

Alembic: made bl_alembic_import_test.py run again

Two tests are still failing, but at least the API changes in 2.8 have been
applied now.