dropbox-cli: Fix and add Nautilus extension

…since this is actually dropbox-nautilus project.

In one of the previous updates, the source code responsible for finding dropboxd
was changed, rendering our sed replacement not working. That triggered the installer
which in turn failed on the unavailability of gobject-introspection typelibs. And
when that was fixed, we got bitten by our lazy packaging putting a broken code
into the installer.

This is a proper fix for all the issues, except for making typelibs available to
the installer since we shall use Nix instead.