Implement Mark/Cursor positioning for tooltips

[dm-p]

Vega-Tooltip has a new feature added where you can prescribe whether to position tooltips relative to cursor (traditional behavior) or marks:

• https://vega.github.io/vega-tooltip/vega-examples.html
• https://vega.github.io/vega-tooltip/vega-lite-examples.html

For us to leverage this, we'll need to:

• Add an option for tooltip management
• Make this a setting that gets exported to templates
• Add similar functionality to the custom Power BI tooltip handler so that this behaves like the Vega one when users set the desired option

As we're currently re-designing the template/project format for v2, we'll factor it into its design and include in the initial v2 versions when we get to re-integrating tooltips and settings.