Fix custom folder icons using an absolute path entry

Summary:
Gwenview does not display a custom folder icon which is configured
by an absolute path and the folder does not contain any images.
This is caused by KIconLoader::loadMimeTypeIcon() failing for
absolute path entries. If the folder contains images the preview
job corrects the icon later.
Using KIconLoader::loadIcon() loads the correct icon.

BUG: 315983
FIXED-IN: 18.04.2

Test Plan:

• Create an empty folder (e.g. /tmp/parent/test-empty)
• Set a custom icon for test-empty using an absolute path
• Start Gwenview in Browse Mode in the parent folder
• Folder test-empty should display the custom icon

Before:	After:

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx, huoni

Differential Revision: https://phabricator.kde.org/D12465