Introduce "OperatingMode"

Introduce "OperatingMode" and better unattended behaviour

Rather than having clients set a range of flags where combinations don't
make sense, expose an enum.

- "SharedAccess". The screen layout from the host is used. Nothing is
locked

- "RemoteAccess" The screen layout is defined by the client. The host
PC is send to the login screen. The session is re-locked on exit.

- "AdditionalDisplay" a single virtual monitor is added with the size
defined by the client. Nothing is locked.

"AdditionalDisplay" mode is only available when manually invoking
krdpserver as that's how kdeconnect works.

The other two are chosen by a simple boolean flag/checkbox