Change the underlying structure of HWND on linux from GdkWindow to
GtkWindow.
Make Makefile.libSwell GTK-aware.
Start implementing owned dialog windows.
Implement dialog windows using GTK+ event boxes.
Store toplevel window as GObject data.
First version with GTK+ menubar (non-functional).
Send WM_COMMAND when activating menu item.
Implement TrackPopupMenu.