Show "Empty Trash" button inside trash directory

Summary:
Show "Empty Trash" button inside trash directory.

FEATURE: 163306

Test Plan:

Reviewers: ngraham, rkflx, markg, elvisangelaccio

Reviewed By: ngraham, markg, elvisangelaccio

Subscribers: markg, emateli, broulik, elvisangelaccio, rkflx, mmustac, ngraham, Dolphin

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D10804