Conference parameters : Change proxy config into account. Deprecate proxy config on linphone_conference_params_get_proxy_cfg().
Conference parameters : add default and custom factory URI, aswell of identity coming from account.
LinphoneAddress : add isValid() to public API.
Create conference : creation from "conference_type" on default account (unchange). If not set, use factory from conference parameters. If factory is not valid then it is a local conference.
Fix const token on account params getter and split conference factory URIs in core to avoid seeking account where it is not needed.
