Add GObject Event Filters

[Michael Janelle-Montcalm]

Currently, we are only able to listen to Gtk events and signals via the GtkEventFilters. We would like to be able to interact with the GObject events too. This means adding a GObject event filter to the core and having a way to identify them uniquely.

For example, the Memorize activity has a Game class that contains the game logic. This class inherits from GObject and implements the application-specific behavior. The use case that promotes this feature is that we should be able to know when the user selects two matching panels.

[Vincent Vinet (vince-vinet)]

branch lp349874 has a little variation of the Widget Identifier that recurses down the activity looking for gobject, listing properties and signals.