As setDesktop was changed to "move" this left unSetDesktop non-symetric.
This replaces it with explicit API to enter/leave.
This also moves new API to the new object based API rather than still
Where numbers are used it has been tidied up so that desktop IDs are
uint, which should be used when we have a list of desktops.
int is used only when we have either a desktop ID or NET::OnAllDesktops
Effects API cleared up to use this and use a set of x11 IDs, which
avoids any potential complications of handling add and removes any
ambiguity with what happens if you leave all desktops and such.