More debug timing info from main RNA diffing func.

Static overrides optimization: 30% quicker.

use stack instead of always allocating memory for RNA paths of checked
properties! From average 167ms to 118ms here with Autumn rig... Still a
lot to improve, but that's already much better.

Merge branch 'master' into blender2.8

Fix usage of lib filepath in previous own commit...

Merge branch 'master' into blender2.8

Add debug tool to check validity of current file *before* saving.

It is hidden behind the --debug-io flag for now.

Idea is to try to catch broken libraries state in current Main before we
actually write the file on disk, should help catching and understanding
what happens in Spring corruption cases.

Finish use-after-free in workbench code.

GHash should probably not own the data itself, but that's other question
to be fixed later, at least this fixes the crash.

Solution by @fclem, thanks!

Disable auto-override for all but active object in group case.

I.E. only enable auto-override for 'active' selected object when making
an override of a linked group. This will ease on auto-override creation,
and you typically do not want to auto-override most objects in the group
anyway (in proxy system, you could only proxyfy one object of the group
anyaway!).

Make depsgraph tag for auto-override IDs it updates.

This will reduce amount of needless auto-override checks, at least when
not touching anything related to overriding IDs...

Cleanup: rename 'static override regarding reference ID' tag.

Just 'OK' was waaayyyyyy too generic!