Switch GCC version for buildbot builds to 10.

BlenRead: Add GHash-based search for already read linked IDs.

Ths commit adds a new `IDNameLibMap` to `Main`, used during file reading
to quickly find already read linked IDs.

Without that, search would use string-based search over list of linked
data, which becomes extremely slow and inneficient in cases where a lot
of IDs are linked from a same library. See also {T89194}.

Extrem-usecase reported in T89194 is now about 4 times faster in linked
data reading (about 2 times faster for the whole .blend file loading).

More normal cases (like Sprites studio production files) have barely
measurable speed improvements, a few percents at best.

NOTE: `main_idmap` API was extended to support insertion and removal of
IDs from the mapping, avoids having to re-create the whole thing several
time during libraries expansion in readcode.

Differential Revision: https://developer.blender.org/D11757

LayerCollections: Add a way to prevent their resync with Collection hierarchy.

This is an easy & safe, yet not-so-nice way to address the
LayerCollections vs. Collections hierarchy resync problem.

Currently this resync is enforced everytime something changes in the
Collections hierarchy, which is extremely inneficient, and can even
produce 'loss' of LayerCollection data during complex Collection
processes.

Current example is during Library Overrides resync process. New code:
* Makes resync significantly faster (between 10 and 15%).
* Fixes 'disappearing' layer collections settings on sub-collections'
layers.

NOTE: This is not a proper fix for the underlying issue. However,
implementing and testing the 'lazy update' solution as proposed by
{T73411} requires a significant amount of time (especially in testing
and tracking all places where code would need to ensure LayerCollections
are up-to-date), which is not possible currently.

Differential Revision: https://developer.blender.org/D11889

Fix i18n utils_cli mistake.

Reported by James Monthea (@jmonteath), thanks.

Fix broken 'undocumented' case in registration of Macro opertators.

Code dealing with macro operators missing description field was slightly
different than the one from Operator registration.

This lead to invalid memory accesses in some python introspection cases
like the i18n messages extraction code in `bl_i18n_utils` module.

Fix some mesh edition macro operators incorrectly setting an empty description string.

No operator or macro should be missing description. But if they do, then
they should use NULL pointer, and not an empty string.

This behavior was already enforced (through an assert) for operators,
previous commit made it the same for macros.

Cleanup: LayerCollection resync: Proper `r_` prefix for return function parameter.

Cleanup: LayerCollection resync code.

Mainly naming (also droping the `layer_collection` in favor of just
`layer` for internal code, this is clear enough and much shorter). Add
proper parent/child identifiers, `r_` prefix for parameters also used as
return values, etc.

Also made some parameters const.

Cleanup: Layer resync: Split object/base resync part in own function.

No behavioral change expected here.

Cleanup: Turn PointCache flags defines into an anonymous enum.