Outliner: Support moving objects from/to group internal collections

Outliner: support Add Selected to group internal collections

Remove "Group Collection" functionality

This may get "double-reverted" in the future. But we want to separate this
from the main patch before merging it. And then eventually merging them
separately.

Merge remote-tracking branch 'origin/blender2.8' into temp-group-collections

Fix collection groups crash after saving and loading file

If you save again and re-open they are still gone though :/

Prevent crash on groups with no collections

Cleanup

Support deleting group collection individual collections

Fix adding new collections to group collections

Show a different Outliner Tools menu option depending on collection type

Silence warning

Unittesting (failing) for read/write

Move group sub-collection evaluation tp DEG

Integrate group PRE processing to depsgraph

Something to check with Sergey:
What happens if the group is instanced AND is in a layer collection?
In this case we need to test something different than LIB_TAG_DOIT for
building groups and their relations.

Improve debug printing for layers evaluation

Assert on IDProp merge routine to simplify debug process

Remove assert from BKE_layer_collection_get_active_ensure

If all collections are group collections, we still need to create a
ew collection to accomodate new objects.

RNA: allow for enable/disable of group collections

Outliner: Support enable/disable group collections and fix select icon

You can select only non-group collections.

Outliner: Support drag & drop of GROUP_INTERNAL collections