Remove KDELibs4Support from KSudoku

Summary:
Port KNetAccess to KIO.
Change signature of Serializer::load, Serializer::loadCustomShape to use QString& instead of QString*
Change signature of Serializer::store: pass errorMsg
Remove unused function.
Port remaining Qt4 calls such as toAscii() or setAcceptsHoverEvents(),

Test Plan: Game can be played, tested loading and saving of current game.

Reviewers: #kde_games, ltoscano, ouwerkerk

Reviewed By: ltoscano, ouwerkerk

Differential Revision: https://phabricator.kde.org/D5627