Work on D-Bus properties etc

Work on D-Bus properties etc

* lisp/net/dbus.el (seq, subr-x): Require.
(dbus-error-disconnected, dbus-error-service-unknown): New defconst.
(dbus-set-property, dbus-register-property): Use `keywordp'.  Fix
proper value sending a signal.

* test/lisp/net/dbus-tests.el (dbus-test04-register-method):
Extend test.
(dbus--test-signal-received): New defvar.
(dbus--test-signal-handler): New defun.
(dbus-test05-register-signal)
(dbus-test06-register-property-emits-signal): New tests.
(dbus-test06-register-property)
(dbus-test06-register-property-several-paths): Rename tests.