Implement CCMP conference scheduler that allows to create a conference

by sending an HTTP message to a CCMP server.
The XML schema of RFC6503 has been slightly modified to overcome a
limitation in the parsing of abstract types  of the xsdcxx library

Do not report ConferenceCallEvent if the call is not linked to any conference or the conference doesn't support media
Prevent registration of dependent account after its removal
Add ENABLE_DEPRECATED_TESTS to avoid running deprecated tests in the CI