BasicManagement: Add a config flag for enabling

Remove the coupling to MediaRenderer: allow BasicManagement on any
rygel plugin, provided the configuration specifies that diagnostics
should be started.

Add a "+DIAGE+" capability that is exposed in the device description
when diagnostics is enabled.

This change means that by default the service will not run. Add this to
rygel.conf to enable:
  [Playbin]
  diagnostics=true
DescriptionFile: create device elements if they do not exists

set_device_element() may be called with names of elements that do not
exists yet (non-required elements that are empty by default).

Create elements dynamically in set_device_element(). also, remove
the X_DLNACAP element if the contents are empty.

https://bugzilla.gnome.org/show_bug.cgi?id=703044