gui/event-editor: Rework calendars menu

Use a GtkListBox and bind from the model. Create rows from the model
items, and bind properties as needed.

The action target is set only when the calendar is writable because
setting an invalid action+target pair makes the row insensitive.

Remove the ancient menu icon hack.

src/utils/gcal-utils.c

@@ -659,75 +659,6 @@ e_utf8_strftime_fix_am_pm (gchar *str,
   return sz;
 }
 
-
-/**
- * fix_popover_menu_icons:
- * @window: a #GtkPopover
- *
- * Hackish code that inspects the popover's children,
- * retrieve the hidden GtkImage buried under lots of
- * widgets, and make it visible again.
- *
- * Hopefully, we'll find a better way to do this in
- * the long run.
- *
- */
-void
-fix_popover_menu_icons (GtkPopover *popover)
-{
-  GtkWidget *popover_stack;
-  GtkWidget *menu_section;
-  GtkWidget *menu_section_box;
-  GtkWidget *widget;
-  GtkWidget *child;
-
-  /* GtkScrolledWindow */
-  widget = gtk_popover_get_child (GTK_POPOVER (popover));
-
-  /* GtkViewport */
-  widget = gtk_scrolled_window_get_child (GTK_SCROLLED_WINDOW (widget));
-
-  /* GtkStack */
-  popover_stack = gtk_viewport_get_child (GTK_VIEWPORT (widget));
-
-  menu_section = gtk_widget_get_first_child (popover_stack);
-
-	/**
-	 * Get the unique box's children.
-	 */
-  menu_section_box = gtk_widget_get_first_child (menu_section);
-
-  gtk_widget_add_css_class (menu_section_box, "calendars-list");
-
-  /**
-   * Iterate through the GtkModelButtons inside the menu section box.
-   */
-  for (child = gtk_widget_get_first_child (menu_section_box);
-       child;
-       child = gtk_widget_get_next_sibling (child))
-    {
-      GtkWidget *child2;
-
-      /**
-       * Since there is no guarantee that the first child is
-       * the GtkImage we're looking for, we have to iterate
-       * through the children and check if the types match.
-       */
-      for (child2 = gtk_widget_get_first_child (child);
-           child2;
-           child2 = gtk_widget_get_next_sibling (child2))
-        {
-          if (GTK_IS_IMAGE (child2))
-            {
-              gtk_widget_add_css_class (child2, "calendar-color-image");
-              gtk_widget_set_margin_end (child2, 12);
-              gtk_widget_set_visible (child2, TRUE);
-              break;
-            }
-        }
-    }
-}
-
 /**
  * get_source_parent_name_color:
  * @manager: a #GcalManager

src/utils/gcal-utils.h

@@ -94,9 +94,6 @@ gsize                e_utf8_strftime_fix_am_pm                   (gchar
                                                                   const gchar        *fmt,
                                                                   const struct tm    *tm);
 
-void                 fix_popover_menu_icons                      (GtkPopover         *popover);
-
-
 void                 get_source_parent_name_color                (GcalManager        *manager,
                                                                   ESource            *source,
                                                                   gchar             **name,