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.
Add C optimizations build output files to .gitignore
utils/misc: Add a filename_from_uri function and use it in timeline previewers
previewers: Add various debugging statements to trace perf problems more easily

+ Make the AudioPreviewer uri a private member and quote it.
previewers: Improve the clarity of rate modifications in PipelineCpuAdapter
Alphabetically sort imports in utils/misc, utils/ui and previewers
previewers: Delay initial thumbnail & waveform processing to when the UI is idle
previewers: Comment and compact code
previewers: Default to 500 ms for the thumbnail interval

This should make thumbnailing feel significantly faster on most computers
check: Make numpy a hard dependency as we now need it for core audio waveforms
effects: Don't error out when the effect doesn't have a recognized media type.
Czech translation
Added Czech language
pipeline: GESTimelinePipeline was renamed to GESPipeline in GES
utils/widgets: Fill sliders with color to indicate the default values

This provides a visual indication of which numeric values the user has changed.