Using `bpy.types.OperatorProperties.__subclasses__()` has become utterly
unreliable now, to the point that it keeps references to freed (aka
unregistered) operators now, leading to crash when accessing them.
This commit refactors quite seriously the `dump_rna_messages` code, by
first listing all 'valid' classes, and then processing them all at once
in a flat iteration.
RNA classes are still generated from class hierarchy rooted on the
'virtual' `rna_struct` one, except for operators. These are now
generated by introspecting `bpy.ops` instead.
b179378766