Fix editing items in SimpleFavoritesModel

Summary:
tools.js is supposed to support both KaStatsFavoritesModel (used for
favorite applications) and SimpleFavoritesModel (used for system actions,
like poweroff/reboot/logout). The latter did not work though, as tools.js
unconditionally calls methods only present in KaStatsFavoritesModel.
This commit makes KaStatsFavoritesModel API compatible to SimpleFavoritesModel
and changes the relevant code in tools.js to support both.

Additionally, this syncs applets/kickoff/package/contents/code/tools.js again.

BUG: 385463

Test Plan:
Before this patch it wasn't possible to remove system actions from
the favorites bar.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: hein, davidedmundson, rikmills, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8178