Since the ::changed implementation of GtkRecentManager implies a
synchronous write operation, when we receive multiple requests to emit a
::changed signal we might end up blocking.

This change coalesces multiple ::changed emission requests using the
following sequence:

  • the first request will install a timeout in 250 ms, which will
    emit the ::changed signal

  • each further request while the timeout has not been emitted
    will increase a counter

      ‣ if the counter reaches 250 before the timeout has been
        emitted, then the RecentManager will remove the timeout
        source and force a signal emission and reset the counter

This sequence should guarantee that frequent ::changed emission requests
are coalesced, and also guarantee that we don't let them dangle for too
long.

https://bugzilla.gnome.org/show_bug.cgi?id=616997