Clean up string casts

Summary:
Compile without casting to/from ascii to avoid typical mistakes due to
implicit conversions. Fix a bunch of dubious places.

Test Plan: Things compile, tests run.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: zzag, anthonyfieroni, broulik, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14060