Add WKnobComposed widget to provide an alternative to pixmap-based rotary knobs.

* Factor out event handling into KnobEventHandler so WKnob and WKnobComposed
  share the same logic.
* Add some more XML utility methods to WWidget.
Fix SVG rendering to scale with higher fidelity.

* Use QSvgRenderer to scale SVG files instead of converting to QPixmap and
  scaling the pixmap.
* Introduces Paintable class that wraps pixmaps and SVG images.
Remove zap_denormal code.

Extremely small values (~1e-38) are slow on Intel, so we had code
to round small values ("denormals") to zero.  But it was causing
waveform discontinuities and therefore pops and clipping with the
new normalized range of -1 to 1.  Furthermore, when we compile in
optimized mode GCC will zap denormals for us anyway, so this code
is unneeded.
Add support for source rectangles when drawing SVGs.
Support styling of WVuMeter with style sheets and make background pixmap optional.
Initialize the butterworth filters with the actual samplerate (rather than hard-coding 44100).
Add timer to EngineFilterBlock::process.
Switch to using float instead of double for butterworth filter coefficients and buffers.

On my machine this shaves 100 microseconds off the min and the max (22% of the
time spent in EngineFilterBlock::process).

With double EngineFilterBlock buffer/coefficients:
Debug [Main]: Stat("EngineFilterBlock::process","count=1735,sum=8.19368e+08ns,average=472258ns,min=435030ns,max=683018ns,variance=5.58157e+08ns^2,stddev=23625.4ns")

With float EngineFilterBlock buffer/coefficients:
Debug [Main]: Stat("EngineFilterBlock::process","count=1663,sum=6.11084e+08ns,average=367459ns,min=333305ns,max=561874ns,variance=9.14966e+08ns^2,stddev=30248.4ns")
Remove 'hifieq' SCons option.

The EQ type is toggle-able at runtime now so it doesn't make sense to keep this
build option.
Don't allocate memory in AnalyserQueue::doAnalysis.
Use unicode with libsndfile on Windows

* Fixes recording and WAV problems with unicode files on Windows
Merge pull request #136 from asantoni/master

Use unicode with libsndfile on Windows
Use constData instead of data in a variety of places.
added generic function to create a new history playlist
Merge pull request #137 from QuentinFaidide/master

added generic function to create a new history playlist
Fix some code style nits
added Quentin Faidide to contributor list, Thank you very much.