These objects are created via e.g. selection_watcher("CLIPBOARD") to
track the CLIPBOARD selection. They start watching when their .active
property is set to true and stop when this property is unset again.
This commit implements the Lua side of that: A list of active watchers
is kept and updated when needed.
The next commit will add the X11-side of this so that these objects
actually do something.
Signed-off-by: Uli Schlachter <psychon@znc.in>
a24f35532c