LibGUI: Close and cancel GDialog on escape

This is a small usability enhancement. If you press escape with a GDialog
focused, it will now return its "Cancel" status.

Libraries/LibGUI/GDialog.cpp

@@ -1,5 +1,6 @@
 #include <LibGUI/GDesktop.h>
 #include <LibGUI/GDialog.h>
+#include <LibGUI/GEvent.h>
 
 GDialog::GDialog(CObject* parent)
     : GWindow(parent)
@@ -40,6 +41,19 @@ void GDialog::done(int result)
     m_event_loop->quit(result);
 }
 
+void GDialog::event(CEvent& event)
+{
+    if (event.type() == GEvent::KeyUp) {
+        auto& key_event = static_cast<GKeyEvent&>(event);
+        if (key_event.key() == KeyCode::Key_Escape) {
+            done(ExecCancel);
+            return;
+        }
+    }
+
+    GWindow::event(event);
+}
+
 void GDialog::close()
 {
     GWindow::close();