There is data that libei and the EIS implementation will want to
exchange that is not covered by the immediate API.

To avoid having to add APIs for all of these, let's provide a generic
property API that both server and client can use to exchange this info.

The property API provides read/write/delete permissions but those only
apply to the client, not the server. The idea is that a server can
create (or restrict) properties that the client can read but not modify
and/or delete. A special-case are properties filled in automatically by
libei: ei.application.pid and ei.application.cmdline. These could be
used by e.g. the portal implementation to match permissions.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>