Summary:
This change introduces support for text input. Text input allows to
compose text on the server (e.g. through a virtual keyboard) and sent
the composed text to the client.

There are multiple interfaces for text input. QtWayland 5.6 uses
wl_text_input, QtWayland 5.7 uses zwp_text_input_v2.

wl_text_input is from pre Wayland-Protocols times and considered as
UnstableV0 in this implementation. The other interface is UnstableV2.
Unfortunately the V2 variant is not yet part of Wayland-Protocols, but
used in Qt.

The implementation hides the different interfaces as good as possible.
The general idea is the same, the differences are rather minor.

This means changes to how interfaces are wrapped normally. On client
side in the Registry a manager is factored which represent either of
the two interfaces. Similar on the server side Display's factory method
takes an argument to decide which interface should be factored. This
way a user of the library can expose both interfaces and thus be
compatible with Qt 5.6 and Qt 5.7 onwards.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1631