Controller
----------

The controller subsystem allows to automate element property changes. It works
so that all parameter changes are time based and elements request property
updates at processing time.

Element view
~~~~~~~~~~~~
Elements don't need to do much. They need to:
- mark object properties that can be changed while processing with
  GST_PARAM_CONTROLLABLE
- call gst_object_sync_values (self, timestamp) in the processing function
  before accessing the parameters.

Application view
~~~~~~~~~~~~~~~~
Application need to setup the property automation. For that they need to create
a GstControlSource and attach it to a property using GstControlBinding. Various
control-sources and control-bindings exists. All control sources produce control
value sequences in the form of gdouble values. The control bindings map them to
the value range and type of the bound property.