Hide file preview when icon is too small

Summary: In the filepicker dialog hide the previews when the icons are too small.

Test Plan:

Reviewers: VDG, Frameworks, ngraham, rkflx, Dolphin, markg, elvisangelaccio

Reviewed By: VDG, ngraham, rkflx, elvisangelaccio

Subscribers: elvisangelaccio, markg, xyquadrat, sharvey, rkflx, ngraham, Frameworks

Tags: Frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D12321