gui: Propagate context to GcalSearchButton

The people who introduced this code were not careful enough to
make sure the code they removed from GcalWindow was added back
in GcalToolbarEnd.

They also simply commented out some code instead of properly
reimplementing that.

I regret having trusted their code :(

Closes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/880

src/gui/gcal-window.ui

@@ -200,7 +200,7 @@
                       </object>
                     </child>
                     <child type="end">
-                      <object class="GcalToolbarEnd"/>
+                      <object class="GcalToolbarEnd" id="toolbar_end" />
                     </child>
                   </object>
                 </child>