dnd: Don't try to remove an invalid idle source

As the handler returns false, the corresponding source is removed
automatically and its id invalidated. Reset the id to 0 to reflect
this, otherwise newer versions of GLib will print a warning when
we later try to remove it explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=711555
dnd: Don't queue an idle handler if we already have one

Removing an existing source before scheduling a new one is not wrong,
but slightly less effective than doing nothing and relying on the
previously created source to do the job.

https://bugzilla.gnome.org/show_bug.cgi?id=711555