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/gcal-event-widget-pool.h

@@ -0,0 +1,39 @@
+/*
+ * gcal-event-widget-pool.h
+ *
+ * Copyright 2026 Georges Basile Stavracas Neto <georges.stavracas@gmail.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include <gtk/gtk.h>
+
+#include "gcal-event.h"
+
+G_BEGIN_DECLS
+
+#define GCAL_TYPE_EVENT_WIDGET_POOL (gcal_event_widget_pool_get_type())
+G_DECLARE_FINAL_TYPE (GcalEventWidgetPool, gcal_event_widget_pool, GCAL, EVENT_WIDGET_POOL, GObject)
+
+GcalEventWidgetPool *gcal_event_widget_pool_new            (void);
+GtkWidget           *gcal_event_widget_pool_take_or_create (GcalEventWidgetPool *self,
+                                                            GcalEvent           *event);
+void                 gcal_event_widget_pool_reclaim        (GcalEventWidgetPool *self,
+                                                            GtkWidget           *event_widget);
+
+G_END_DECLS