Add support for progress slider widget

[jus]

This is a request to extend the existing <SliderComposed> widget functionality to allow for sequential <Slider> background images, like we have it in the <Knob> widget.

This would combine the best from both worlds. <Slidercomposed> works horizontal or vertical and allows scaling [1], while <Knob> can load sequential images.

Usage
-------
* A progress bar can indicate, well progress, if space is tight e.g. replacing the small waveform overview and its tiny play position indicator.
* A progress bar slider can improve visibility by a factor. Currently it is hard to see the actual position of the line/crossfader handle at a glance.
* A progress bar slider can also replace a rotary knob, if necessary.

Example (crossfader)
----------------------------
old: ---|-----------------
new: ---|=======----------

Demo
--------
http://bootstrapmaster.com/live/simpliq2/ui-sliders-progress.html

[1] https://github.com/mixxxdj/mixxx/pull/426

[Ferran Pujol (ferranpujol)]

I've implemented a different approach for WKnobComposed in PR #645 (https://github.com/mixxxdj/mixxx/pull/645).
I added an additional Pixmap that is masked according to the position of the knob.

Would something like this suit your needs?

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/7959