[Property] Add "object.item" upnp class type

This is a port of commit https://github.com/01org/media-service-upnp/commit/97351d7963aa2b6bf3f72a1c45f8bae9f195ed7f

Signed-off-by: Regis Merlino <regis.merlino@intel.com>
[Build] Enable SOUP session User Agent Customization

- Add the optional --with-ua-prefix flag to allow the customization
of the SOUP session User Agent Customization at build configuration time.

- Construct our user agent prefix with the following template:
%s dLeyna/%VERSION

%s is the value provided --with-ua-prefix if any
%VERSION is the dLeyna version

- Call g_set_prgname(...) API to make our ua-prefix usable by libsoup.

Example:

with ./bootstrap-configure --with-ua-prefix=MypProject user agent value
will be:
  MyProject dLeyna/0.0.1 GUPnP/0.19.4 DLNADOC/1.50

if ./bootstrap-configure is called without --with-ua-prefix, user agent
value will be:
  dLeyna/0.0.1 GUPnP/0.19.4 DLNADOC/1.50

This is a port of commit https://github.com/01org/media-service-upnp/commit/22c051228d484392a42803e3e841819ea8789782

Signed-off-by: Regis Merlino <regis.merlino@intel.com>
[Device] Handle unsubscribe when exiting

Signed-off-by: Regis Merlino <regis.merlino@intel.com>