timeline: This adds a marquee which allows to select ... nothing.

Yay.
timeline: On dragEnd, get the set of toplevel elements contained

in the lasso selection.
utils/timeline: remove unneeded parameter "other".

	+ timeline/elements: And remove it in the instantiation of course.
timeline/elements: Adds support for grouping / ungrouping.
This commit adds a waveform audio previewer.

	+ previewers: Adds an audio previewer.
	+ elements: Add this previewer as a child of audio elements.
	+ Adds a new folder "coptimizations"
	+ Adds a new C extension, renderer, which renders the
	  samples on a cairo surface.
	+ Makefile.am:
	+ /* Fallthrough */
	+ configure.ac:
	+ autogen.sh:
	+ Updates to compile the renderer.
previewers: implements a pipelineCPUAdapter class.

	+ And use it in waveforms.
Handle different number of channels. Order samples according to their stream time.
previewers: add saving and loading for the waveforms.

	+ They are pickled in ~/.cache/pitivi/waves
Regulate the frequency at which thumbnails get created.

	+ With respect to the CPU usage.
Make the waveform pipelines go to READY when rate < 0.1.
Fix make check and make distcheck with waveforms.