gcal-application: Ctrl+R to synchronize calendars

In !332, there was some difficulty with having two accelerators for the
"Synchronize calendars" action, so the Ctrl+R accelerator was left out.

This commit adds the Ctrl+R accelerator to gcal-window.ui by specifying
it before the F5 accelerator. The result is that only F5 is shown as an
accelerator in the overlay menu, but Ctrl+R still works.

Ctrl+R was also added to help-overlay.ui, displayed alongside F5.

Fixes #1052

src/gui/gcal-window.ui

@@ -98,6 +98,12 @@
             <property name="action">action(win.show-calendars)</property>
           </object>
         </child>
+        <child>
+          <object class="GtkShortcut">
+            <property name="trigger">&lt;Ctrl&gt;r</property>
+            <property name="action">action(app.sync)</property>
+          </object>
+        </child>
         <child>
           <object class="GtkShortcut">
             <property name="trigger">F5</property>

src/gui/gtk/help-overlay.ui

@@ -24,7 +24,7 @@
             <child>
               <object class="GtkShortcutsShortcut">
                 <property name="title" translatable="yes" context="shortcut window">Synchronize calendars</property>
-                <property name="accelerator">F5</property>
+                <property name="accelerator">F5 &lt;Ctrl&gt;r</property>
               </object>
             </child>
             <child>