(core) Brings in the new donut charts.

Summary:
  - Donut charts is same as pie chart with few extra options to control size of the hole and to show/hide a big total in it.
  - Add a new option type to tune a numeric options using a slider/spinner/keyboard.
  - Add a new option type to tune a numeric options using a slider/keyboard
  - Add a new .propWithDefault method to ObjObservable to allows to set a default value when options is undefined.
  - mocha-webdriver's findContent does not work to find content in svg elements. So had to tweak original function into a sister function using .textContent instead.

Test Plan: Adds new tests

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: anaisconce, dsagal

Differential Revision: https://phab.getgrist.com/D3107

static/ui-icons/Charts/ChartDonut.svg

@@ -0,0 +1,56 @@
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