gui/window: Set parent widget as last focus when event widget is unfocusable

When selecting an event with a pointer, the keyboard-focused widget
was being set instead of the last focus widget, which was
then refocused after the event popover closed.

This patch fixes that bug by only setting the parent widget as
the last focus widget if the event widget itself is unfocusable,
like in the agenda view. The `GtkWidget:focus-on-click` property
is already enabled, so it takes care of refocusing the event
widget if it's focusable.

Commit https://gitlab.gnome.org/GNOME/gnome-calendar/-/commit/a3fb57939a307f5b661cc2f205e562f43a0a31fa
technically fixed https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/915,
but this patch solidifies the commit.

Closes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/915

src/gui/gcal-window.c

@@ -772,6 +772,18 @@ day_selected (GcalWindow *self)
   update_active_date (self, gcal_view_get_date (GCAL_VIEW (self->date_chooser)));
 }
 
+static GtkWidget*
+find_first_focusable_widget (GtkWidget *widget)
+{
+  GtkWidget *aux = widget;
+
+  while (aux && !gtk_widget_get_focusable (aux))
+    aux = gtk_widget_get_parent (aux);
+
+  g_assert (GTK_IS_WIDGET (aux));
+  return aux;
+}
+
 static void
 event_activated (GcalView        *view,
                  GcalEventWidget *event_widget,
@@ -779,7 +791,7 @@ event_activated (GcalView        *view,
 {
   GcalWindow *self = GCAL_WINDOW (user_data);
 
-  g_set_weak_pointer (&self->last_focused_widget, gtk_root_get_focus (GTK_ROOT (self)));
+  g_set_weak_pointer (&self->last_focused_widget, find_first_focusable_widget (GTK_WIDGET (event_widget)));
 
   gcal_event_widget_show_preview (event_widget, event_preview_cb, user_data);
 }