Make thumbnail frame-and-shadow drawing criteria match those of the file dialog

Summary:
KIO's file dialog already has logic to avoid drawing frames around images detected as likely to be icons, which is improved with D15071. Since Dolphin doesn't use KIO for any of this (boo) we have to pull it over here too; this patch brings the same feature to Dolphin, as well as the feature to disable frames and shadows for all thumbnails at very small sizes, which improves clarity.

With this patch, Dolphin's frame drawing behavior becomes consistent with that of the file dialog (as of D15071).

BUG: 295526
FIXED-IN: 18.12.0

Test Plan:
Icons no longer have frames:

Images without transparency still have frames:

Nicer presentation for folders with mixed image types (images without transparency get frames; images without it don't):

At small sizes, thumbnail clarity is improved by omitting the frame and shadow. Before:

After:

Reviewers: Dolphin, broulik, elvisangelaccio

Reviewed By: Dolphin, broulik, elvisangelaccio

Subscribers: markg, anthonyfieroni, elvisangelaccio, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D15069