Fix GTK build
Use control packet for mouse drag/drop and hover

This removes some more pre-assembled C4Script code going across the
network unchecked.

Part of #936.
Remove C4ControlScript::fInternal

fInternal basically acted as a reverse "evil bit" as in RFC 3514: when
set, the engine would not do any checks on the script contained in the
control packet, nor log the script (visibly in game; the packet log
would of course contain the packet). A malicious game client thus would
be able to inject arbitrary script without people (immediately) noticing
anything was amiss.

As of this patch, only the host is able to execute arbitrary scripts,
and those will be shown in the message board for all players to see.
This privilege can be irrevocably disabled in network games by any
client by using the "/nodebug" message board command.

Closes #936.