core: add dynamic password infrastructure

Some media sources (e.g., DAAP) support notifying a client that a password is required at connection time. Since such sources are commonly detected with protocols like DNS-SD, Grilo needs a mechanism to support handling such password requests.

This work adds a new "grl_source_notify_authenticate" function which a plugin can call when a password is needed. The function, in turn, emits an "authenticate" signal which the application should respond to by obtaining a password. The application then provides the password back to the plugin by calling the new "grl_daap_source_continue_with_password" function.

Signed-off-by: W. Michael Petullo mike@flyn.org