Implement multi-dragbox

Implement multi-dragbox

--name _a_
	make a dragbox called _a_; using --name, added items go to
this dragbox, --get requests from this dragbox. This allows multiple
instances running. The current instance name will be shown in the
window title.

--list
	list the currently available dragboxes by their internal
names. This will list the internal names (those are modified from the
user-given names, only english alphabet lowercase letters allowed)

Updated manual page as appropriate

Note that we could have chosen to use one process for all dragboxes.
Here we simplify greatly by using org.gna.dragbox.identifier as the
service name, and use one service and process per window. Otherwise it
would have been _much_ more of a rewrite.