gui: Introduce GcalEventWidgetPool

This is a helper object that preallocates a number of event widget,
and then gives them away to consumers. When consumers are finished
using the event widget, instead of simply destroying it, they can
return the widgets to the pool, so it can be further reused.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/758>

src/gui/meson.build

@@ -47,6 +47,7 @@ sources += files(
   'gcal-drop-overlay.c',
   'gcal-event-popover.c',
   'gcal-event-widget.c',
+  'gcal-event-widget-pool.c',
   'gcal-fading-label.c',
   'gcal-meeting-row.c',
   'gcal-overflow-bin.c',