mount-operation: add a ShellMountOperation implementation

Ideally, this would be an entirely-JS implementation, but we have a
couple of issues with gjs and gobject-introspection to work around, so
we need a ShellMountOperation class for the time being.

This first commit implements the show-processes dialog, with a system
modal style very similar to the EndSession dialog.
Implementations of ask-question and ask-password will follow shortly.

https://bugzilla.gnome.org/show_bug.cgi?id=653520