Blender Git Loki

LibOverride: Fix last main issue with overriding custom properties.

Now, custom props defined as overriddable can be overridden, saved,
reloaded, etc. That fixes the last main issue with them.

Note that custom props still have a lot of glitches and weirdness in
their overriding behavior, but for now the most important is finally
achieved, will let them rest and settle a bit, those have been
incredibly painful to tame... :(