Implemented support for hide/show groups

Summary:
Added an option on PlacesPanel context menu to show or hide the entire
group of places.

Depends on D8855

Test Plan: Open Donlphin and use PlacesPanel context menu to hide and show groups

Reviewers: franckarrecot, mlaurent, mwolff, elvisangelaccio

Reviewed By: franckarrecot, mlaurent, mwolff, elvisangelaccio

Subscribers: rkflx, mwolff, elvisangelaccio, ngraham, #dolphin

Differential Revision: https://phabricator.kde.org/D9242