Blender Git Loki

Make Scene Master collection 'Private' ID data, like root nodetrees.

Same issue here as with root nodetrees, those are private ID data owned
by another ID, and not in Main DB. This requires special handling.

there are still quiet a few things to do here, like getting rid of
special code for master collection (regular ID copying should handle
that just as it already does for root nodetrees), cleanup in ID copying
code, etc.