Add a show keyframes button which calls a show Keyframes method on the relevant timeline element
This commit makes addition of keyframes possible.

Conflicts:
	pitivi/timeline/elements.py
	pitivi/timeline/timeline.py
Usability work.
Rendering: port to 1.0.

	+ Remove audio depth as it doens't exist anymore
	+ Rename other caps.
	+ Use RENDER instead of SMART_RENDER as it's broken.
project: Remove dead code.
project/timeline: Port to new Track API
elements: When the track is an AUDIO track, add volume keyframes.