Code refactorings for MicroViewSlider

Replaced if/else blocks with switch, not sure if the originals where so because they could have been faster. Submitting for consideration anyways.

Simplified MicroViewSlider::draw() to remove redundant code.
I'm talking about changes to the **MicroViewGauge::draw()** method in the **Gauge** class, *not* the **Slider** class.

Look at lines 1707 - 1718. They are duplicating twice the code in lines 1696 - 1700 that you added.

Your changes in the **Slider** class are fine.