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