map-feature bug fixes

[AliyanH]

• Removing map-feature to leave an empty layer- throws an error #838
• make click/focus/blur events work and in sync with the g element (can work as attribute, API, and addEventListener).
• Allow prevention of default click handling, can be done as following:

// $0 represents the map-feature element$0.addEventListener("click",(e)=>{e.originalEvent.stopPropagation();});

• add support for keyup/keydown event on map-feature
• Add Tests

closes#838

[prushforth]

Good simplifications, and they seem reliable.

[prushforth]

I would have liked to get rid of the click(ev), focus(ev) and blur(ev) parameter, as it seems non-standard, but perhaps that's not possible because we have got leaflet handling the underlying clicks and displaying popups and so on.

[AliyanH]

noticed another issue just now that I will work on, removing a map-feature when it is unchecked does not work (can be handled in a separate issue).

[AliyanH]

I think this is good to go @prushforth , just a few changes I added:

• removed non-standard parameters for click, focus, blur methods (based on suggestion by @prushforth)
• Added support for keyup/keydown on map-feature

[prushforth]

Would you mind fixing this bug in this PR? If that doesn't make sense, feel free to merge this, it looks and works well.

[prushforth]

I like the test for preventDefault that displays a different thing than the popup for the polygon, good idea there. It tests the API and not just our API; that's exactly what we need to do more of.