Add ability to define dot size for line visualisation

[tommikiviniemi-srs]

Kibana version:
5.4.3

Elasticsearch version:
5.4.3

Describe the feature:

The line drawing widget already allows you to define line width and to toggle dots on and off. However, there is no way of defining dot size, which adds a glaring discrepancy between line widgets, Timelion and Visual Series Builder widgets, as the latter two allow defining dot size.

[ghudgins]

For Lens: Closing this in favor of #124279 which has a few more details / requirements.

This support was added to Aggregation-based Visualizations in ~7.16 🎉