FlatLaf update should happen on EDT

I have run into some issues with LAF changes in the fiji chat UI. It seems we might need a defensive check that ensures FlatLaf calls run on the EDT

I tried doing this a couple years back with 0f08dd4 (set-laf-on-edt branch), but it never made its way onto mainline; I don't remember why not. Perhaps it didn't fix the issue, or needed more testing?