Be strict about implicit string casts

Summary:
Use QT_NO_CAST_FROM_ASCII and friends. There are a bunch of places where
QString should really be QByteArray, but that's for the future.

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: zzag, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14065