Animation data (including drivers) on inactive view layers now work. The
removal of such view layers was too optimistic; they are now kept
around. The bases are still removed, mostly for safety sake and to keep
the changes to a minimum.
`scene_remove_unused_view_layers()` has been renamed to
`scene_minimize_unused_view_layers()` to reflect its new functionality.
For compatibility with assumptions in other areas of the code, the
function still ensures the input view layer is at index 0.
This also introduces a new function
`BKE_view_layer_free_object_content(view_layer)`, which is a subset of
the functionality of `BKE_view_layer_free()`.
After remapping there is no guarantee that collections' parent pointers
are still valid, so using these to free the object cache of all the
collections' ancestors is potentially accessing invalid memory.
Further more, it is noticiably inefficient, as a same collection may be
processed many times.
So instead, introduce (and use in resync case) a new
`BKE_main_collections_object_cache_free`, which iterates over all
collections of given Main and only free their own cache.
