BeatMap: Fix bpm and sync behavior with beatmaps.

YOUR BPM INDICATOR IS LYING TO YOU.  Since a beatmap
doesn't have a constant bpm, using the "file_bpm" value
for sync and slider display is misleading.  This commit introduces
a new ControlObject, "local_bpm", which represents the average
bpm value around the closest 8 beat span.  It's still a fairly
noisy value, so it may require tuning.  This local_bpm
value is used to display the visual_bpm, so now you can
watch the bpm of your crazy track change over time.  Also it's
used for master sync, so you can watch the pitch slider
of your track wobble around as it tries to stay in sync.

TESTED=existing tests pass, and some new tests for the bpm averaging function.
clarifying comment
notes
Better support for SVG slider handles.

* Scale WSliderComposed handle to the width or height of the widget.
* Update Deere slider handles to look more like classic Deere -- still
  missing the dropshadow.

Raster handles are usually custom-built to be the same width/height as
the slider so this should not affect most existing skins.
Add slider backgrounds modeled after classic Deere.
Merge pull request #423 from rryan/slider

Better support for SVG slider handles.
Merge pull request #419 from ywwg/mastersync-remove-filebpm

BeatMap: Fix bpm and sync behavior with beatmaps.
Disable log spam for the (common) case of no beatgrid with quantize enabled.
LateNight: Move buttons around so that they don't move when they are clicked.
Fix math error in source rectangle for horizontal VU meter peaks.
Deere fixes.

* make knob line ends round
* SVG "rgba" is not supported by Qt. Use stroke-opacity and fill-opacity instead.
Rename EffectChain parameter to super.

Named the EffectChain control 'super1' for expansion. Updated Shade,
Deere, and LateNight. Fixes Bug #1378211.
Remove DOS line endings.
Update ControlPickerMenu for superknob CO change.
Remove loading of resource path from mixxx.cfg.

Fixes issues with using multiple versions of Mixxx on the same
machine. Bug #1392854.
Process cover art when loading tracks from browse mode or external libraries.

Partial fix for Bug #1393749.
Remove connection to non-existent signal.
When processing covers on first scan, cache the results before doing the
real work to prevent an active SELECT from blocking the database.

Fixes Bug #1399981.
Deere skin: Add simple Load/Save Samplerbanks buttons to the samplers extended view. This allows to load/save groups of tracks to sample decks. Fixes https://bugs.launchpad.net/mixxx/+bug/1181215
Change .gitignore to track *.cfg files in /res/keyboard/