This commit makes PieChartView adhere to the UIAccessibility container
protocol. See the Accessibility section marked for the methods. To allow
conformance, 3 properties are added to IPieChartDataSet and
PieChartDataSet that enable a more user friendly audio descriptionfor
data elements. PieChartRenderer has a new private method
createAccessibleElement() that populates the accessiblePieChartElements
property, which in turn is used in PieChartView. Note that to prevent
contextless audio descriptions, the default Chart Description header
text was deleted in Description.swift since it is already an optional.

Minor changes to spacing in PieChartRenderer.swift. Removed formatting
and use of "self." to match library style.

Updated ChartViewBase, ChartData and ChartDataRendererBase to declare
the primary properties required for accessibility support within the
Charts library. This includes the UIAccessibility protocol methods
within ChartViewBase and the internal accessibleChartElements property
in the base renderer. ChartData also has 3 optional properties to allow
proper formatting of audio.

Added accessibilityChildren to ChartViewBase which is a layer over both UIAccessibilityContainer and NSAccessibilityGroup protocols. Updated PieChartRenderer to use the platform agnostic NSUIAccessibilityElement. Added init() overrides in NSUIView declaration for macOS to add .list NSAccessibilityRole. Added Platform+Accessibility.swift which extends NSUIView with accessibility container and group protocols and also declares NSUIAccessibilityElement, which acts as an abstraction over NSAccessibilityElement and UIAccessibilityElement.

…lity

Moved Platform+Accessibility.swift to Utils folder. Changed
accessibleChartElements to be final since Renderer subclasses should not need to
modify its working. Simply populating it in draw() functions will add
basic accessibility.

Created internal property accessibilityOrderedElements to make
BarChartRenderer be composed of logically ordered accessible elements (See inline comments for details). Updated ChartDataRendererBase, PieChartRenderer and Platform+Accessibility with updated comments to reflect the platform agnostic NSUIAccessibilityElement's use.

Updated HorizontalBarChartRenderer to populate
accessibilityOrderedElements, which in turn is used by the
BarChartRenderer superclass to enable accessibility. Added minor note to
BarChartRenderer's createAccesibleElement() for edge case where x-axis
labels can be inaccurate if multiple data sets are present.

LineChartRenderer now has a private accessibilityOrderedElements nested array that is then used to populate accessibleChartElements. Do note that the nesting is unnecessary for now, but will be needed once a custom rotor is added. Also unlike most other renderers, LineChartRenderer's accessibleChartElements is populated in drawCircles(). This required moving the check for isDrawCirclesEnabled() after accessibleChartElements are populated.

Updated BubbleChartRenderer to mirror LineChartRenderer's nested
use of accessibilityOrderedElements to populate accessibleChartElements.
Minor updates to comments in LineChartRenderer.

Updated RadarChartRenderer with accessibility properties and calls in
drawData and drawDataSet. Due to the unique spatial arrangement of radar
charts, accessibleChartElements is populated by dataset, within which
variables are ordered in decreasing order.

Fixed crash due to incorrect use of maxEntryCountSet instead of dataSetCount
in BubbleChartRenderer and LineChartRenderer's
accessibilityCreateEmptyOrderedElements(). Updated BubbleChartRenderer's
bubble accessibilityLabel to include percentage size of bubble based on
maxSize property of IBubbleChartDataSet.

Updated CandleStickChartRenderer to populate accessibleChartElements.
Unlike most other renderers with multiple dataSet support, we do not
attempt to order elements logically and hence dataSets are presented to
VO in the same order they are drawn with the dataSet label acting as a
separating heading.

Added a workaround for non updating accessibility frame when resizing
windows and using Charts on macOS by using setAccessibilityFrameInParent() in Platform+Accessibility. See inline comments for details.

Added createAccessibleHeader() to ChartRendererBase.swift that is used
to create a descriptive header for all subclasses based on dataSet count
and labels. RadarChartRenderer and PieChartRenderer updated to add
dataSet label to each item and with documentation respectively.

Added an offset to BarChartRenderer's barRect calculation in prepareBuffer() to prevent
calculation of rects outside visible chart area, while still allowing
automatic offset of the axis minima visually. This workaround is only
used when using auto calculated y-axis minima and isn't used when a custom
minimum is manually set.

Updated return value for the index(of:) function to be NSNotFound in
Platform+Accessibility's iOS section. This is as required by the
documentation for UIAccessibilityContainer protocol.