Safari does not always trigger "loadingdone" event (actually it rarely does) and thus in most cases we end up with unloaded fonts on init and paste. It's especially annoying when there are tons of font faces to load, such as with the recent CJK support and half or even less characters remain unloaded.

The solution is loading all the necessary font faces manually in Safari, both on init and when pasting elements.

Fixes #3229

Other improvements

• Consolidates font faces logic in export under the Fonts class
• Concurrently loads 10 font faces at a time (loading all 250 font faces at the same time likely isn't a good idea)

TODO

• Regression test export
• Double-check font faces are not loaded twice or more times
• Double-check we don't load font faces which shouldn't be loaded
• Load only font faces for pasted elements

Follows #8530