[provider] requires contextkit pkgconfig modules

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[subscriber] subscribe after signal is connected

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