Clean-up DolphinFacetsWidget

Summary:
Small refactor patch to clean-up some logic on setter and getter:

• Search terms are previously splitted and set separately, so no need for extra splitting and foreach loop
• Return search terms on a QStringList rather than join first

Depends on: D26029

Test Plan: No behavior changes

Reviewers: Dolphin, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D26150