demo: Mostly implement org.freedesktop.FileManager1

As we can open folders we can also provide that DBus interface. With this things like "open containing folder" in Firefox can work thus easing testing the dialog and giving me a chance to practice a bit more Rust. Note that we're not doing ShowItemProperties atm as that needs another dialog which would somewhat fit into pfs though so I might add it at some point.

Closes: #16 (closed)