Use user-trash icon for Trash action

Summary:
BUG: 391078
FIXED-IN: 19.04.0

Test Plan:
Before:

After (WITHOUT QT_SCALE_FACTOR=2 gwenview)

(WITH QT_SCALE_FACTOR=2 gwenview)

Reviewers: ngraham

Reviewed By: ngraham

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D18391