Change state handling.

 - Combine current state, target state and flags in one struct.
 - Remove notification types, simply notify for every state change.
Move restart handling to lib.
Limit notifications actual changes in state.
Fix DEBUG define.
Set object context ref in p1_object_init.
Better notification and state handling.

 - Extra flags that can inhibit start based on config and deps.
 - Objects get a chance to handle others' notifications.
 - Video mixer now properly links/unlinks based on notifications.