Blender Git Loki

Collections: preserve exclude flag of child collections when unexcluding

Excluding a collection also changes the exclude setting on all child collections
so that it is possible to selectively enable some children without the parent
being enabled.

This change makes it so that if you unexclude the parent, the exclude setting
of children are restored again instead of being permanently lost.

Original patch by Szymon with modifications by Brecht.

Differential Revision: https://developer.blender.org/D7016