Add symlink emblem to thumbnails in thumbnail mode

This is a more traditional "symlink emblem as an overlay" approach to adding symlink icons to thumbnails (compared to !50 (closed)), like how Thunar and file managers do it.

I think the only disadvantage is that instead of directly using a thumbnail from .cache, it uses the thumbnail to create a new pixmap (to combine the symlink emblem). I guess this could increase processing and take a little more time, but I doubt it's measurable so it's probably okay.

I tried to do some scaling to the symlink emblem so it gets smaller with smaller thumbnails (so as not to block 16px svg previews for example). That may need to be refined in the future but so far I haven't seen any problems using multiple icon themes.

The other part to !50 (closed) was adding the symlink info to the details which this MR doesn't do, but I'll try to make another MR for that in the future.

Let me know how it looks/works and if there should be any changes! And sorry for taking so long for the followup! Thanks!