Label improvements

[boygirl]

This PR adds improvements for labels and builds on the label background feature work in #1583

This PR includes several breaking changes related to label and tooltip layout. Most of these changes will be minor, visual changes, but style updates may be necessary.

A more complete list of changes: (Full documentation of new and changed props ~Monday)

• adds support for backgroundStyle for VictoryLabel
• adds support for backgroundPadding for VictoryLabel
• adds a labelPlacement to VictoryPie. Possible values are "vertical" "parallel" and "perpendicular" as in polar labels.
• corrects labelPosition on VictoryPie (previously startAngle and endAngle were inverted) This is a breaking change
• improves text size approximation (this is a breaking change as it alters the automatic heights and widths of tooltips. In most cases this should be desirable)
• adds a flyoutPadding prop to VictoryTooltip to give finer control over spacing between label and flyout edges
• correctly applies tooltip theme props to tooltips in all chart types this is a breaking change
• Alters tooltip themes to add flyoutPadding this is a breaking change
• adds support for polarAxis, polarDependentAxis and polarIndependentAxis namespaces in themes. These are merged with other Axis namespaces
• adds a polarDependentAxis theme to VictoryTheme.material this is a breaking change
• reduces label padding in boxplot and candlestick themes this is a breaking change
• Alters material and grayscale default label font so that they use universally available fonts
• absolutely positions all polar axis elements relative to the origin rather than rendering them all into a group, and translating the entire group. This change corrects issues related to rendering polar axis labels in VictoryPortal