Cut down on boilerplate by using the (no longer that) new helper
macros. We don't care about breaking ABI in private libraries, so
use G_DECLARE_FINAL_TYPE even where the class struct used to be
exposed in the header, except for types we inherit from ourselves
(obviously) or where the class exposes any vfuncs (where changes
could affect inheritance in extensions).
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
