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/gui/event-editor/gcal-event-editor-dialog.ui

@@ -24,7 +24,26 @@
             </child>
             <child type="title">
               <object class="GtkMenuButton" id="sources_button">
-                <property name="popover">sources_popover</property>
+                <property name="popover">
+                  <object class="GtkPopover">
+                    <style>
+                      <class name="menu" />
+                    </style>
+                    <child>
+                      <object class="GtkScrolledWindow">
+                        <property name="propagate-natural-height">True</property>
+                        <property name="width-request">250</property>
+                        <property name="max-content-height">600</property>
+                        <property name="hscrollbar-policy">never</property>
+                        <child>
+                          <object class="GtkListBox" id="calendars_listbox">
+                            <property name="selection-mode">none</property>
+                          </object>
+                        </child>
+                      </object>
+                    </child>
+                  </object>
+                </property>
                 <property name="sensitive" bind-source="GcalEventEditorDialog" bind-property="writable" bind-flags="default|sync-create" />
                 <property name="tooltip-text" translatable="yes">Select Calendar</property>
                 <style>
@@ -171,8 +190,6 @@
       </object>
     </child>
   </template>
-  <object class="GtkPopoverMenu" id="sources_popover">
-  </object>
   <object class="GtkSizeGroup">
     <property name="mode">vertical</property>
     <widgets>