[Feature] Expose mouse events on legend area or improve mouse events on legend

[robin-gerling]

What problem does this feature solve?

Improve user experience of a selection in the legend by using double click on a single series:

• to select a single series (if all series are selected), and
• to select all series (if some series are not selected).

This simplifies the UI (because a workaround using legend.selector adds additional buttons and has differentiating functionality) and improves the user experience (with legend.selector: "inv", the user has two steps instead of a single dblclick to get to the state of selecting a single series).

What does the proposed API look like?

• In case of exposing mouse events: Mouse events are only exposed in the grid area of the chart. They should be also exposed in the legend area. Proposed changes: Add query: "legend" to https://echarts.apache.org/en/api.html#echartsInstance.on. The event parameters should then include the clicked series.
• In case of new legend functionality: No API necessary (could potentially add option to legend whether dblclick should be enabled)