Add Trash (empty, isEmpty, emptinessChanged)
Summary: Add Trash class to handle all trash operations.
Reviewers: elvisangelaccio, markg, ngraham
Reviewed By: elvisangelaccio, markg, ngraham
Subscribers: ngraham, markg, rkflx, elvisangelaccio, Dolphin
Differential Revision: https://phabricator.kde.org/D11012