Support custom button(s) in chart panel

[febbraiod]

We had a case where we wanted to have a button drawn within the chart itself to offer a popup using Hoist's modal support (without have to include a toolbar or a panel header).

Was unable to render said button with our icons (even when converted to an svg using asHtml: true).

Not 100% sure whether or not this idea is supported by highcharts but I was unable to crack it in what seemed like a reasonable amount of time for the ask, but Lee and I didn't want us to lose the idea.