Cleanup warnings

[MichaelAquilina]

Listed when running make

• [x] Fix bug in vala 0.38
• [x] warning: method `Synapse.BansheeActions.BansheeAction.action_available' never used
• [x] rhythmbox-plugin.vala:100.7-100.42: warning: method `Synapse.RhythmboxActions.RhythmboxAction.action_available' never used
• [x] zim-plugin.vala:170.19-172.11: warning: unreachable catch clause detected
• [ ] unhandled error `GLib.Error'
• [ ] Gtk.Stock has been deprecated since 3.10
• [ ] Gdk.Device.grab has been deprecated since 3.20
• [ ] Gdk.Display.get_device_manager has been deprecated since 3.20
• [ ] Gdk.Window.set_background_rgba has been deprecated since 3.22
• [ ] Gtk.Widget.override_background_color has been deprecated since 3.16
• [ ] Gdk.Window.process_all_updates has been deprecated since 3.22
• [ ] Gdk.cairo_create has been deprecated since 3.22
• [ ] Gdk.DeviceManager.get_client_pointer has been deprecated since 3.20
• [ ] Gdk.Device.ungrab has been deprecated since 3.20
• [ ] Gdk.Screen.get_monitor_geometry has been deprecated since 3.22
• [ ] Gdk.Screen.get_monitor_at_point has been deprecated since 3.22