Implement Table, change MasterProxy accordingly.

The Table and TableEntry classes are now implemented.

The MasterProxy class now doesn't handle tables any more. Instead, a
table registers its variables with a MasterProxy object. In other words:
the Table knows its MasterProxy, but not vice versa.

In addition, the MasterProxy now supports adding/removing several
variables at once (handy for table entries) and provides an
isRegistered() method.
Add DisplayStringVariable class.