Bug fixes, new features and better examples

Features:
==================
- Partial VisualState support: Normal, Disabled and Focused.
- All the control parts can now have custom brushes and some of them
have editable Opacity values.
- Input prompt when tapping the middle can now be disabled. This is
especially useful for small controls where you easily click the middle
and open the digit input.
- VisualStates example added to example application.
- Default slider color is set to the PhoneAccentBrush. Can be overriden
by the developer with any sort of brush.
- Code optimizations and restructuring. Regions added to the
RadialSlider.xaml.cs so that it's easier to find what you're looking
for.
- and more...

Fixes:
==================
- Missing unscubscribing from the TouchFrameReported event when the
control unloads. Now added.
- When tapping the slider value textbox, input prompt appeared but
sometimes the slider was too sensitive and moved. Fixed by adding a
check for Action type in the Touch_FrameReported event.
- Now when the control is disabled, its values cannot be changed.
- and more...