[server] Text input content hint and purpose per protocol version

Summary:
Convert content hint and purpose according to used text-input protocol version.

This way we can use different specifiers, which enables us in the future to use
the default hint of v0 and the pin purpose of v3.

Also code is cleaner when using the correctly versioned specifier names according
to the protocol version.

Test Plan: Compiles, autotest passes.

Reviewers: #kwin, #frameworks, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D16708