Use the new drag handle in the Language KCM

Summary:
This makes the list items draggable.

To do this, the backend model was split up into three models so
the move() method could actually move a row as the new drag handle
expects. This also makes move and remove actions a little bit
faster since there's no proxy abstractions involved. The use of
Plasma.SortFilterModel with JS callbacks for filtering has been
dropped from the QML code.

The lists now use DelegateRecycler to be speedier.

The list of available languages is now guaranteed to be sorted in
a locale-aware manner and case-insensitively, using QCollator.
This likely works better than whatever PSFM was doing with the
Qt::DisplayRole sort role before.

Also drops a stray unused KConfigGroup member from
TranslationsModel.

Reviewers: mart, davidedmundson

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13194