Port the dbus-glib-based examples to GDBus.

Also, don't use libnm in them at all; there's not much point in
examples that use the D-Bus API directly if they're just going to fall
back to libnm for the hard stuff... (And also, this avoids the problem
that GDBus uses GVariant, while the libnm-core APIs currently still
use GHashTables.)

Also fix up some comment grammar and copyright style, and add emacs
modelines where missing.

Also rename the existing GDBus-based examples to have names ending in
"-gdbus", not "-GDBus", since there's no reason to gratuitously
capitalize here.