* disconnect/reconnect signals around move/resize. Direct method call is not a solution - setGeometry is called from too many places
* removed the screen < 0 check as that cannot happen.

Introduce a proper screen property in Toplevel

Instead of calculating the screen number each time screen() is invoked,
the screen number gets stored in a private member variable and evaluated
whenever either the screen count changes or the Toplevel's geometry
changes. During move/resize the screen property doesn't get updated. The
update is delayed till the end of the move/resize operation.

Testscript:

var clients = workspace.clientList();
for (var i=0; i<clients.length; i++) {
  var client = clients[i];
  client.screenChanged.connect(function () {
    print(client.screen);
  });
}

moved windows around