[server] Put some text-input v0 exclusive callbacks in v0 class

Summary:
Requests

* reset
* commit_state
* invoke_action

are used in text-input v0 only and neither in v2 and upcoming
v3 of the unstable text-input protocol.

To increase readability put their callbacks in v0 interface class only.

Test Plan: Compiles

Reviewers: #kwin, #frameworks, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D16673