[server] Put set surrounding text callback with uint in v0 class

Summary:
The set surrounding text request is only in v0 of text-input with uint arguments,
in v2 and v3 the call has the same signature.

To increase readability put the uint version in the v0 class implementation
and rename callbacks.

Test Plan: Compiles

Reviewers: #kwin, #frameworks, zzag

Reviewed By: #kwin, zzag

Subscribers: davidedmundson, zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D16675