Port some code away from AbstractClient::setDesktop() to AbstractClient::setDesktops()

The main motivation behind this change is to reduce the number of usages
of int-based virtual desktop apis. int-based desktop ids are highly
unreliable. For example, if a new virtual desktop is inserted in the
middle or removed in the middle, the desktop ids will change. This makes
working with virtual desktops code more challenging due to its behavior.
This is not an issue with VirtualDesktop objects.