Add support for keyboard layout switching policy "winclass"

Summary:
This is quite similar to the policy "window" - the main difference is
that windows from the same application share the layout. So only
switching to a window from another application changes the layout.

This change is the last policy to add for support of all the policies we
have on X11.

Reviewers: KWin, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5365