Add an option to supress the display of the numeric value for widgets

The slider and gauge widgets make it quick and easy to graphically display values within a range, but I feel the displaying of the numeric value along with the graphic sometimes limits them.

These changes allow a flag, **WIDGETNOVALUE**, to be added to the style parameter, which results in only the graphic part of the widget being drawn. The numeric value is not drawn.

For example, to create a slider of style 2 without the numeric value, you could use:
```c++
MicroViewWidget *slider2 = new MicroViewSlider(8, 4, 0, 100, WIDGETSTYLE2 + WIDGETNOVALUE);
```
If desired, you could then add your own code to display the numeric value wherever and however you like.

This request is based on, and includes, the code refactoring that **esmitperez** submitted in pull request #17, although some of it has been replaced by further refactoring.

I've also made a few other changes in this pull request:
- Fixed a compiler warning caused by **emilong**'s circleFill clipping changes from pull request #14.
- Added a destructor method to the **MicroViewWidget** base class to prevent a compiler warning if a widget object is deleted.
- Changed the gauge **drawFace** method to skip the drawing minor ticks over major ticks.
- Further refactored the widget pointer code by adding private methods to replace duplicated code.

I've also written a sketch, **MicroViewWidgetDemo**, that demonstrates many widget possibilities using the new flag. I'll submit this as a separate request.