ws: Add cockpit_web_server_get_document_roots() function

This is a handy accessor to get the already resolved expanded
root document paths from the web server.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
pkg: Simpler AMD module ID calculation

Simplify the calculation of AMD loader IDs, and finding paths to
where to load other modules from. In particular, remove hard coded
base paths.

Then take away the 'cockpit/' prefix, in our module ids. It was
repetitious and made it hard for tests to run in our checkout tree.

Closes #1507
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Allow calls without arguments without introspection

Calls without arguments can proceed without introspection data. In
addition default to zero arguments when a DBus call is made without
arguments.

Closes #1510
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
shell: Hide everything but top bar until ready

The shell initializes asyncronously, it waits for package loading,
and soon it will wait for AMD modules as well.

Only show the very top bar until we're ready to start interacting
with the user.

Otherwise we see very ugly things like a half drawn dashboard before
jumping to another screen.

Closes #1509
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>