Create a placeholder protocol for missing parts of text-input-unstable-v4

As we are implementing support for text-input-unstable-v4, there are requests and events that are missing from the new version of the protocol that we depended on in v2.

Anything we were unable to work around by using the v4 protocol should be wrapped into a special protocol which can serve as documentation of what is missing. This could also be upstreamed if we decide so, as a service to users who want to use the v4 protocol and still get the full functionality with Qt apps. Although, the qt-specific text input protocol might be sufficient for this use case, since it maps everything one-to-one, so the most important purpose of this would be to have an overview of what would be required to fill the gaps in v4.