[kcms/virtualdesktops] Properly track synchronizing state

When the user applies a change like adding a desktop
DesktopsModel::syncWithServer is called and m_synchronizing is set to
true. The server then sends some kind of response (e.g. desktopCreated)
and updateModifiedState is called which sets m_synchronizing to false
again.

However if a setting is changed that does not trigger any server change
(e.g. the show OSD setting) m_synchonizing is never set to false again,
causing problems down the line.

Instead of relying on updateModifiedState to reset m_synchronizing track
the syncronizing state by refcounting the DBus calls

BUG: 437466
(cherry picked from commit 7f36f012)