LibGUI: Check if event loop is alive before quitting it in Dialog::close

Previously we would quit the event loop of a dialog without checking if
it is still alive in the Window::close overload. This patch updates the
implementation to make use of the existing done method, handling closes
more gracefully.

This fixes a CommandPalette crashing when opening an about dialog, as
since 1074c399f3 the command palette
dialog would handle a WindowBecameInactive event after closing itself
due to the action already being called.

Userland/Libraries/LibGUI/Dialog.cpp

@@ -116,7 +116,7 @@ void Dialog::event(Core::Event& event)
 void Dialog::close()
 {
     Window::close();
-    m_event_loop->quit(ExecCancel);
+    done(ExecCancel);
 }
 
 }