Convert timeline debug feature to use the logging framework

Version 3:

• renamed new_subscriber to new_subscription
• added destroy_subscription
• 'timeline' scope makes use of both of the callbacks to create a 'timeline_subscription' object for each subscription
• series ID as it is no longer necessary, because each subscription will have its own IDs
• objects will have a list 'timeline_subscription_object'.
• 'timeline_subscription_object' will be created if the object was not seen before, and checked before emitting a new object ID.

Version 2:

• use a custom_write in the scope instead of adding it (the callback) in the subscriber

Depends on !144 (merged) to be merged as it depends on the logging framework. It attempts on addressing #142 (closed). (!144 (merged) was merged)

This a continuation of @pq work introduced in b502654b where the timeline has been added to weston to debug latency issues.

Adding these now to gather some feedback if the design is appropriate.

This is mostly copy-pasta with the notice that I've added a new callback in the subscriber class to allow passing va_list number of variable number of arguments. Once this was settled everything when straight in: we create a timeline type of subscriber and create a new timeline scope per compositor.