CKitProperty improvements

only is_first_access_ is used, no need in is_initialized_

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[bridge] empty value if subscription is failed

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
qt parent-child for qt objects lifecycle control

also parent_ member in ProviderBridge is redundant
Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
cosmetic changes

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