This commit changes a lot of how IDs are handled internally, especially the unlinking/freeing
processes. So far, this was very fuzy, to summarize cleanly deleting or replacing a datablock
was pretty much impossible, except for a few special cases.
Also, unlinking was handled by each datatype, in a rather messy and prone-to-errors way (quite
a few ID usages were missed or wrongly handled that way).
One of the main goal of id-remap branch was to cleanup this, and fatorize ID links handling
by using library_query utils to allow generic handling of those, which is now the case
(now, generic ID links handling is only "knwon" from readfile.c and library_query.c).
This commit also adds backends to allow live replacement and deletion of datablocks in Blender
(so-called 'remapping' process, where we replace all usages of a given ID pointer by a new one,
or NULL one in case of unlinking).
This will allow nice new features, like ability to easily reload or relocate libraries, real immediate
deletion of datablocks in blender, replacement of one datablock by another, etc.
Some of those are for next commits.
A word of warning: this commit is highly risky, because it affects potentially a lot in Blender core.
Though it was tested rather deeply, being totally impossible to check all possible ID usage cases,
it's likely there are some remaining issues and bugs in new code... Please report them! ;)
Review task: D2027 (https://developer.blender.org/D2027).
Reviewed by campbellbarton, thanks a bunch.
We cannot nuke treestore in readfile's `blo_lib_link_screen_restore()`, because this will
destroy all UI-state data (like opened/closed items, etc.).
Since we cannot know for sure whether an ID pointer from tselem->id is valid here, we
have to ensure they are never invalid, i.e. to always set them to NULL when we delete them.
To do so, this commit uses a similar approach as what already exists for ID references
in WM notifiers - it extends `free_notifier_reference_cb()` to also nullify those IDs in
all outliners.
Note that some ID types are not used(shown) by outliner currently, so `TREESTORE_ID_TYPE` macro
was added, that checks whether an ID is possibly used by outliner. Avoids a few searches
in whole tree whene deleting some IDs.
Reviewers: campbellbarton, sergey
Maniphest Tasks: T44439
Differential Revision: https://developer.blender.org/D1272
