Support border color and border width for pie chart slices

[ndr3]

PR Checklist:

• [x] I have tested this extensively and it does not break any existing behavior.
• [x] I have added/updated examples and tests for any new behavior.
• [x] If this is a significant change, an issue has already been created where the problem / solution was discussed: N/A

PR Description

Support border color and border width for pie chart slices

Add support for displaying borders around the pie chart slices (custom border width and color, similar to Bar charts).

It looks like there are other people who need this functionality (see for example the discussion from https://stackoverflow.com/questions/38758885/border-around-pie-chart-in-mpchart) The same feature was implemented on iOS too in this Pull Request https://github.com/danielgindi/Charts/pull/4168.

[aflorant]

@PhilJay +1 Really need this or my graphic designer will kill me ;)

[nissimr]

@PhilJay +1 would help a lot for empty slices with just a stroke around it and transparent fill