window: implement removing and undoing the removal of a calendar

We're still facing the bug where the just-added calendar segfaults when removed.

data/ui/source-dialog.ui

@@ -100,6 +100,7 @@
                     <property name="receives_default">True</property>
                     <property name="halign">end</property>
                     <property name="margin_top">6</property>
+                    <signal name="clicked" handler="action_widget_activated" object="GcalSourceDialog" swapped="no"/>
                     <style>
                       <class name="destructive-action"/>
                     </style>

data/ui/window.ui

@@ -145,10 +145,7 @@
                         <property name="can_focus">True</property>
                         <property name="receives_default">True</property>
                         <property name="no_show_all">True</property>
-                        <signal name="clicked" handler="undo_remove_event" object="GcalWindow" swapped="no"/>
-                        <style>
-                          <class name="text-button"/>
-                        </style>
+                        <signal name="clicked" handler="undo_remove_action" object="GcalWindow" swapped="no"/>
                       </object>
                       <packing>
                         <property name="left_attach">1</property>