Legend Widget Options

[roikle]

While testing the legend widget, I thought of few changes/additions I'd find handy if included.

Suggestions:

1. Provide an option to include a legend title. I envision this as a string which would be placed at the top of legend if provided in the widgets.json file, otherwise the default would be to exclude the legend title.
2. By default it would be nice if an atlas builder could control the order of the legend items, and if by default it was both alphabetical and also showing features by geometry type (eg. point features on the top, then line features, and on the bottom polygon features). item 2 was addressed by Legend widget update #651
3. It would also be nice if the legend supported grouped graduated symbols and proportional symbols
   map example