Summary:
This change introduces the initial support for keyboard layout switching
policies like in the X11 session. This first change only adds support for
Global and Virtual Desktop policy. This means the current layout is
stored in context to the current virtual desktop. Whenever one changes
the virtual desktop the previous layout is restored. If the user has not
yet navigated to this virtual desktop a switch to default layout is
performed.

This is the first code interacting with the new Virtual Desktop API which
is not based on integer ids. To fully support this the API is slightly
extended.

Test Plan: Added test case

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5301