Plumb menu item activation events from WindowServer to clients.

GMenu now has an "on_item_activation" callback that fires whenever one
of its items are activated. The menu item identifier is used to distinguish
between items.

Use this to implement font switching in Terminal. :^)

Applications/Terminal/Terminal.cpp

@@ -749,3 +749,11 @@ void Terminal::flush_dirty_lines()
     }
     update(rect);
 }
+
+void Terminal::force_repaint()
+{
+    for (int i = 0; i < m_rows; ++i)
+        line(i).dirty = true;
+    m_need_full_flush = true;
+    update();
+}