Smoother bookmark link loading animation

[flaksp]

In commit 74f1facf2e46234772ed58160dbe9bed13f39f27 I've added bookmark link loading indicator. But this background animation appears very suddenly. It would be great to add something like 0.5-1.0s opacity transition to make Camellia feel smoother.

It's impossible to use transition with background-image, so this improvement needs some solution research.