Nested async calls are always a challenge.
Hopefully, things are straightened out now,
and we report GTK_DIALOG_ERROR errors for
the cases we care about.
Just relying on GAppInfo leads to suboptimal
results. Instead, call either the OpenURI portal
or the org.freedesktop.FileManager1 interface
directly, and only fall back to GAppInfo.
The wrapper code for the OpenURI portal is taken
from gio, with small adjustments.
Fixes: #5260
