Clean-up DolphinFacetsWidget

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