* Bind the foreign-toplevel-manager object *after* the first round of
global objects. This ensures we bind all pre-existing wl-output
objects before binding the toplevel manager. This is important, since
otherwise we wont get any output_enter() events for the initial set of
toplevels.
* Bind xdg-output-manager, to be able to bind xdg-output objects for
each wl-output.
* Add xdg-output-listener to each wl/xdg-output, to be able to get the
outputs’ names.
* Add a list of outputs to each toplevel. The output_enter() event
adds to this list, and output_leave() removes from it.
* Add option ‘all-monitors’. When not set (the default), toplevels are
only included in the generated content if they are mapped on the same
output as the bar itself. When *not* set, all toplevels are always
included in the generated content.
