[Enhancement] [WPF] ActivityIndicatorRenderer to use circular animation

[peterfoot]

Summary

Currently the WPF implementation of ActivityIndicator uses an indeterminate ProgressBar which doesn't really make sense in most scenarios. Windows 8 and above have used a circular spinning dots animation for this purpose although it doesn't exist as a Win32 or WPF control. I have implemented a custom renderer (https://github.com/inthehand/InTheHand.Forms/blob/master/InTheHand.Forms/InTheHand.Forms.Platform.WPF/ActivityIndicatorRenderer.cs) to achieve this consistent look using a very simple WPF UserControl which is modelled on the WinRT/UWP control. I'm proposing a Pull Request to replace the current renderer with this one which provides a better UI.

API Changes

No API changes are required to the control. The renderer fully supports the IsRunning and Color properties of the ActivityIndicator control. This change will affect the appearance on the WPF platform only.

Intended Use Case

The control is intended for showing indeterminate activity and on other platforms uses some variant of a circular spinning device. If you have designed a Xamarin Forms UI with the control a stretched ProgressBar will look out of place (and there is already a separate control for that).