appsrc: Add new max-buffers / max-time / current-level-buffers / current-level-time / leaky-type properties
These work the same way as the corresponding properties on queue and allow to control the internal buffer size of the appsrc in a more flexible way.
Unlike in queue the max-buffers and max-time properties are 0 (i.e. disabled) by default for backwards compatibility reasons.
Edited by Sebastian Dröge