[server] Put text-input (de-)activate, en-/disable callbacks in child classes

Summary:
The activate/deactivate and enable/disable callback functionality was in the
TextInputInterface::Private interface, but these calls are each specific to v0
and v2 of the protocol.

Since v3 will have again a different function signature, put all the callbacks
and their helper functions in the protocol version specific child classes.

Test Plan: Compiles, runs.

Reviewers: #kwin, #frameworks, davidedmundson, zzag

Reviewed By: #kwin, davidedmundson, zzag

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D16676