need to port/create: on-screen keyboard

Keyboard used currently and the one likely to be provided on the devboards is weston-keyboard. It does its job, but it's rather unsuitable for a phone.

We need a design which is easier to modify and prettier, with support for multiple languages, ideally written in GTK3.

A possible solution is eekboard, which is already GTK3, but has a rather unnecessary server-client DBus interface, and is not aware of Wayland input method.