Add support for multiple application windows

[joansalasoler]

Support for multiple windows was disabled because Clutter crashes with a GLXBadDrawable error when more than one window is open. It would be nice to add this feature back.

[joansalasoler]

GTK: 3.24.20
Clutter: 1.26.4
OS: Ubuntu 20.04.1 LTS
Graphics: Mesa Intel HD Graphics 530 (SKL GT2)

(auale:22274): Gdk-ERROR **: 12:46:47.369: The program 'auale' received an X Window System error.
This probably reflects a bug in the program.
The error was 'GLXBadDrawable'.
  (Details: serial 10133 error_code 160 request_code 152 (GLX) minor_code 29)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)