they should have implemented native dialogs when they had the choice!
muwahahaha

this is currently limited to firefox and not applied globally because I
am certain it won't be working with everything GTK. so, to prevent random
breakage only firefox for now (which is the single gtk app we ship
by default).

subject for evaluation come 20.04; I fear for 18.04 portal tech in general
is too new to expect everything to work properly.

this relies on portal support being fully provided, which is enabled via
neon-desktop. might move somewhere lower in the stack even, it's a pretty
central part of the containment story.

BUG: 403739