util: cKitValueDefault()

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
"cap" const for readability

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[subscriber] a bit smarter property update

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>