Load thumbnails on main screen asynchronously

[TheLastProject]

Decoding a bitmap can be slow. On slow devices, loading all the images as soon as the card should be shown can lead to UI freezes.

By loading the thumbnail asynchronously, scrolling quickly remains smooth even on slow devices.

Note: This works quite well on my Fairphone 3 with just a few images, needs testing with many images and also on fast devices.

[TheLastProject]

Hmm, this one might be controversial.

It does make scrolling smoother, but you may also see the text instead of the icons for a second, even when opening the app (and this seems to also happen on relatively fast devices like Samsung Galaxy S20+ 5G).

I'm not sure if it's an improvement :/

[TheLastProject]

Nobody ever complained about slow scrolling, so let's just keep this open in case this becomes a problem ever (so we have the fix ready) but leave it the way it is for now.