Fix crash when accessing id->newid in some cases.

ID->newid was not cleared on reading file, since until now it could be
saved with whatever random value...

Merge branch 'master' into id_newid_optional

Merge branch 'master' into id_newid_optional

Cleanup id->newid usage, initial work.

This aims at always ensuring that ID.newid (and relevant LIB_TAG_NEW)
stay in clean (i.e. cleared) state by default.

To achieve this, instead of clearing after all id copy call (would be
horribly noisy, and bad for performances), we try to completely remove
the setting of id->newid by default when copying a new ID.

This implies that areas actually needing that info (mainly, object editing
area (make single user...) and make local area) have to ensure they set
it themselves as needed.

This is far from simple change, many complex code paths to consider, so
will need some serious testing. :/