Add DBus interface to OrientationSensor and a persistent configuration

Summary:
The initial state for user enabled is now read from config. In addition
a DBus interface is provided exporting this property. This allows
KScreen to determine whether automatic screen rotation is available and
whether the user has it enabled or not.

Furthermore KScreen can change the property and this gets stored into
the configuration. Thus KScreen can offer a user interface to
enable/disable automatic screen rotation as well as enabling/disabling
the manual rotation based on the current user settings.

Test Plan:
Not yet tested, coded on the system which doesn't have an
orientation sensor

Reviewers: KWin, Plasma, sebas, davidedmundson

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D8738