Each tray client is directly reparented to the bar window. This saves us
the hassle of having to configure a basically useless tray window and
keeping its background pixmap in sync.
The only disadvantages are having to (un)map each client window
individually and calculating its position relative to the bar window
(which changes all the time) instead of relative to the tray window
(which only changes when clients are added/removed).
tray_client class contains a reference so copy assignment operator is implicitly deleted.
the dtor unembed the window so copying the class could lead to bug so the copy ctor is also deleted.
Fixes#1674
