window: Add win.show-requests action

And assign it to the Requests button, which is now a toggle button and
allows toggling between showing only normal contacts or only requests in
the contacts side pane.
tox: Add pending-requests property

This will be used to keep track of how many open friend requests there
are.
tox: Don't leak contacts when destroying
tox: Add neuland_tox_remove_contact()

Now we can properly remove contacts. Only works for requests at the
moment, because the UI for deleting normal contacts isn't functional
yet.
window: Fix two GBindings

These are per window!