Add context to translated strings and clarify some strings
Summary:
Many of the shorter strings are not really informative, so add context
so translators can figure out what they mean. Some of the strings were
also wrong, those have been corrected as well.
Test Plan: Still compiles.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D29721