Add support for additional echart events

[falkoschindler]

This PR tries to solve feature request #2679 by adding support for additional chart events. They can be accessed using the "chart:" prefix like this:

ui.echart({
    "brush": {},
    "xAxis": [{"type": "value"}],
    "yAxis": [{"type": "value"}],
    "series": [{"type": "scatter", "data": [[1, 2], [3, 4]]}],
}).on('chart:brush', lambda e: print(e))

This example prints the current selection when using the rectangle or box selection.

Open tasks:

• documentation
• pytest