[wayland] Always have a keyboard on the Seat

Summary:
This is a workaround for QTBUG-54371 resulting in QtWayland never
requesting the input methods panel without having keyboard focus.
Thus also the virtual keyboard is not working.

With this change we go back to always announcing a keyboard and
binding the virtual keyboard to whether we don't have an alpha-numeric
keyboard instead of whether there is a keyboard on the seat.

Reviewers: KWin, Plasma on Wayland, bshah

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2343