Instead of trying to query GAppInfo for the default inode/directory
handler, use org.freedesktop.FileManager1:ShowItems with a hole poked
in the sandbox to permit access.

Turns out that libdazzle already provides a convenient wrapper around
org.freedesktop.FileManager1:ShowItems for a single URI. This is as
good an excuse as any to start using libdazzle and dropping the
copy-pasted src/egg-*.[ch] code.

#7