Use empty/filled style for monochrome empty/full trash

Summary: We shouldn't rely purely on color to indicate state and red is the wrong color in the first place.

Test Plan:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23705