"Save as defaults" for chart rendering options

[rtapella]

At the bottom of the right panel, add two new buttons: "Save as defaults" and "Revert defaults to built-in"

• Save as defaults would save the current rendering options as the default ones for this user/session:
  dot size (scatterplot)
  opacity (scatterplot)
  dot shape (scatterplot)
  trend line (scatterplot, time series)
  show grid lines (scatterplot, time series, single-x-multi-y)
  number of bins (histogram)
• Revert defaults would reset all those values (for the current chart type) back to the built-in default