Merge branch 'master' into soc-2019-openxr

Merge branch 'master' into undo-experiments

Merge branch 'master' into asset-engine

Note: reworking filebrowser UI is still TODO.

Conflicts:
release/scripts/startup/bl_ui/space_filebrowser.py
source/blender/blenloader/intern/versioning_280.c
source/blender/editors/include/ED_fileselect.h
source/blender/editors/space_file/file_draw.c
source/blender/editors/space_view3d/space_view3d.c

Fix T69789: Assert when create a new Full Copy scene base on 2D template.

Private ID data (nodetrees and scene collections...) need special care
and handling of their copy flags, and checks must be adapted too.

In that case, issue came from the fact that even though those IDs have
to be copied outside of bmain, we may still require usercount handling.

That commit also fixes a somewhat related issue - we cannot use the
non-id private data copying flag for private IDs copying, due to
difference in handling of usercount again.

I18n: dirty workaround for PITA ghost classes remaining after unregister.

Spent again a whole day trying to understand what happens here, with no
luck. For some reasons, OperatorProperties children of unregistered
operator classes remain accessible, with their `bl_rna` member, although
that one is 100% invalid (freed memory, crashes with ASAN builds).

Funny thing is, I cannot reproduce that situation when disabling the
add-on from the py console of a Blender-with-UI.

Note: issue revealed by X3D add-on, which is still enabled in factory
settings, while not being officially supported any more, this has to be
fixed in a separate commit.

Fix (unreported) non-official X3D add-on enabled in factory startup.

Forgot to remove it from there when I moved it to community-supported
status last week.

Also raising subversion.

Fix (unreported) crash after new mapping node commit.

Blatant obvious usage of freed memory in rBbaaa89a0bc54.

And also fix a memleak in same code/commit...

LibOverride: give more remapping control to `BKE_override_library_create_from_id()` too.

Similar change to the one done for tagged IDs overriding some days ago.
We do not always want to remap all local usages of a linked data-block
to its new local overriding copy.

LibOverride: add a generic macro to check whetehr an ID is overridable.

...and use it in code generating library overrides.

LibOverride: Initial attempt to ignore missing place-holders IDs.

Ideally, when a reference linked ID is missing (and replaced by linking
code with an empty place-holder), we should just keep the local
overriding datablocks as-is, until broken links are fixed.

Not really working yet though, needs more work here...