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.