Virtual operator=() is IMO pointless, and especially in a class hierarchy
like SdrObject it's pretty unlikely one could reasonably assign any
SdrObject-based object to any other one. Moreover, it was actually
only used in Clone(), which was almost never reimplemented, so the
more sensible choice is to have non-virtual operator= and virtual
Clone() always being reimplemented and using that.

This commit also fixes various smaller or bigger, er, interesting
details in the various operator= implementations.