P1Context inherits from P1Element.

Also make p1_set_state and p1_set_target proper methods of P1Element.
P1Element -> P1Object; P1PluginElement -> P1Plugin.
Implement context stopping.

Context state handling now also uses the target field.
Implement stopping and freeing of audio mixer.
Release locks while waiting for blocking calls.
Don't hold locks for processing callbacks.
Some remaining renaming.