html: Fix batch response handling in TorcWebsocket
libtorc-core: Add TorcHTTPService::GetProperty
libtorc-core: Support sending positional parameters in TorcRPCRequest
libtorc-core: Send the property value when notifying property changes.

- this simplifies interaction with the server.
- complex properties should be shadowed by a version property (or
similar) - as in TorcMediaSourceDirectory - that is notified instead of
the actual property.
html: Fix event handlers in TorcWebsocket
libtorc-core: Use a map for properties in TorcHTTPService

- this then translates to an object in json/javascript which is easier
to manage.