Reorder members in ManagedVTable

This is to prepare splitting it up into a
several type dependent vtables.

Change-Id: I5de8234e40ffc73fab47d43f4a1b30d244fc6ad9
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Adjust method naming to be closer to Managed

Also make markObjects() virtual, to be in line with
Managed.

Change-Id: I3e7682216660e2027c02c9181e541b12310902f3
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>