Blender manpage generator: use blender build date instead of current time.

It makes much more sense to use the build timestamp of the Blender
binary used to generate that manpage, than the current time.

As a bonus, when Blender building makes use of the SOURCE_DATE_EPOCH envvar
(through CMake, since previous commit), this also propagate automatically
to that man page.

Inspired by D5756 by Bernhard M. Wiedemann (@bmwiedemann), thanks.

Cleanup: Manpage: use 'https' blender.org URL instead of 'http' one.

BLI_tasks: simplify/generalize heuristic computing default chunk size.

That code is simpler and more general (not limited to some specific
values of thread numbers). It still gives similar default chunk size as
what we had before, but handles smoother increase steps, and higher
number of threads, by keeping increasing the chunk size.

No functional change expected from that commit.

Cleanup/sanitize a bit Action copying within `BKE_animdata_copy()`.

Whole animdata copying code would need much more love, but we can as
well sanitize some pieces of it when we hit them, for now.

Fix T69931: Materials with keyframes duplicated by 'make single user' are linked.

Another sneaky bite from the infamous private ID data: While those
monsters are not in bmain, the actions used by their animdata are
regular cute little ID's, living with the herd in the safe and sound
Main DB...

So we have to be careful not to propagate the nasty black magic
required to handle the formers when we duplicate their animdata.

Saying it again: private ID datablocks should never have had their own
animdata & actions, this is endless issue also with RNA paths... And
makes copying of animation between materials and such needlessly
complicated.

Fix T64238: Scene Full Copy: Cameras bound to markers does not update.

That's exactly why we should get rid of all those 'custom cases'
remapping code, it's hard enough to keep a single place
(library_query.c) up to date and 100% valid, but having more areas doing
their own remapping is just impossible to maintain... Some day...

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.