import-dialog: Show event count in header bar

Fixes a regression introduced in !465 by replacing the
`GtkHeaderBar` with a `AdwHeaderBar`

Closes #1265

src/gui/importer/gcal-import-dialog.c

@@ -44,7 +44,7 @@ struct _GcalImportDialog
   GtkWidget          *cancel_button;
   AdwComboRow        *calendar_combo_row;
   AdwPreferencesGroup *files_group;
-  GtkHeaderBar       *headerbar;
+  AdwHeaderBar       *headerbar;
   GtkWidget          *import_button;
   GtkWidget          *placeholder_spinner;
   AdwPreferencesPage *preferences_page;

src/gui/importer/gcal-import-dialog.ui

@@ -7,8 +7,9 @@
     <child>
       <object class="AdwToolbarView">
         <child type="top">
-          <object class="GtkHeaderBar" id="headerbar">
-            <property name="show-title-buttons">False</property>
+          <object class="AdwHeaderBar" id="headerbar">
+            <property name="show-end-title-buttons">False</property>
+            <property name="show-start-title-buttons">False</property>
 
             <!-- Cancel button -->
             <child type="start">
@@ -93,3 +94,4 @@
   </object>
 
 </interface>
+