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:     |
| {F5818845} | {F5818847} |

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx, huoni

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