Show resources thumbnails in the quick load menu

[mieldepoche]

This is what the quick load menu (in the inspector, RMB on a resource field > quick load) shows when looking for a texture:

it's the same for all resources: only the resource type icon is shown, no thumbnail.
This forces you to parse a long list of files.

The solution is clear: show resource thumbnails. But how?

1. show the thumbnail instead of the icon, using a bigger image size, keeping the current list view (so, thicker items) ?
2. show a grid view of the thumbnail and file names? (con: can't show full path)
3. allow toggling between those two options?
4. ?

[timothyqiu]

See also #3788