elements: don't edit the clip if it wasn't actually moved.

Fixes https://github.com/pitivi/pitivi/issues/99
clipproperties: After (de)activating an effect, commit the timeline.
waveforms: the sync property of the sink must be set to True.

Otherwise rate modulating was just useless.
pitivi: Clear the timeline when closing projects

Timeline was not cleared when closing the project / creating a new
project

https://github.com/pitivi/pitivi/issues/103
project: Add some debug infos
pitivi: Be less verbose when we can not get pipeline position

In some cases it is just normal, so do not bother the user about it.
previewer: Cleanup imports
bin: Do not forget GES in PKG_CONFIG_PATH
mediafilespreviewer: Always use the same videosink

Avoiding races where the sink is disposed too soon
pitivi: Disable the transformation box for now
project: Sample rate are integers, cast them

https://github.com/pitivi/pitivi/issues/97
ui: Do not mixup bitrate and samplerate
timeline: Start cleaning UI elements when they are removed

Fixes https://github.com/pitivi/pitivi/issues/105
pipeline: Do not add 2 times timeout for seeking
transitions: Bring back transition type handling