[applet] Add "Show in File Manager" action when opened

After recent changes to the actions, when you open a vault, there is no standalone action to show it in a file manager. Normally this is not needed if you use the "Unlock and Open" action, but if you use that when then close it by mistake, or use the "Unlock" action that does not automatically spawn a file manager window, then it may not be obvious how to actually get to the vault.

Accordingly, this commit adds a standalone "Show in File Manager" action that appears in the expanded view, but only when the vault is currently unlocked.

cc @teams/vdg