[KRunner] Avoid writing history if addToHistory is idempotent
Summary:
It is not uncommon to run the same command repeatedly. In this case,
the history is actually unchanged - the item is removed from the first
position, and prepended again.
Test Plan:
run the same command twice, config file is not rewritten
run a new command, config is updated
Reviewers: Plasma, broulik, apol
Reviewed By: Plasma, broulik
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D20383