Move ConversationsSortFilterProxyModel class from conversationlistmodel files to its own files

## Summary

Currently `OurSortFilterProxyModel` class is implemented in the same file with `ConversationListModel` class, which would likely grow in the future.

This MR separates `OurSortFilterProxyModel` class into its own separate file renaming it to `ConversationsSortFilterProxyModel`

https://invent.kde.org/kde/kdeconnect-kde/-/merge_requests/229