[core] choose isolate on registration

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
master pointer for QJSEngine in actor

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[actor] allow also null as request callback

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
invokable methods are public

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
link core with dl

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