pathbar: Extend button menu with extensions and templates

One of the benefits of the new menu on the path bar buttons is that we
show the background menu.

This was the intended design since the start, but we didn't come to
finalize it earlier on.

Now with the 3.30 approaching, this work implements that.

Closes #405
1 job for master in 1 minute and 37 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed #66821
flatpak

00:01:37