meson: get filebrowser plugin working again

This won't work on Windows but it's good enough for now.

This functionally matches what autotools did:
cat the 2 mkenums templates together. It feels a
bit messy to me but it seems to make sense.
1 job from master in 8 minutes and 44 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #197983
flatpak

00:08:44