For some windows we don't want to create a PlasmaWindow. Not all
ShellClients are something the outside world should see. This change
introduces the first restrictions:

• KWin internal windows are hidden
• transients not accepting focus are hidden

The latter case doesn't work though if the Surface is mapped prior
to creating the shell surface. In such a situation it's racy as KWin
handles the create surface request before we get the setTransient
request. This is difficult to handle as we do want to react quickly.