Potentially use SVG <title> over aria-label + Leaflet Tooltip methods

[Malvoz]

<featurecaption>s are realized by:

1. Setting aria-label on SVG <g> elements to provide the accessible name
2. Enabling Leaflet tooltips (duplicating the aria-label, to be displayed on hover)

The SVG <title> element (as child of <g>) can be used to achieve both of the above. The native tooltip from <title> is visually similar - if not identical - to the title attribute.

Pros of SVG <title>:

• Reduced code
• Performs better than JS based tooltips
• No "magic"! (i.e. same consideration as for popups: Replace popup implementation with popup event #403). MapML can instead specify that <featurecaption> should be treated similar to the SVG <title> element and/or the HTML global title attribute (in regards to displaying a native tooltip on hover)
• Potentially avoid double speak in the future: If Leaflet were to ever make tooltips accessible to screen readers (i.e. apply role="tooltip" or Live Region ARIA attributes) and we pull in that release then we'd have both the tooltip and aria-label announced by screen readers.

Cons:

• Not possible to style native tooltips
• Does not actively follow the cursor once displayed
• Is not displayed on focus (only hover) 😕

[Malvoz]

• Is not displayed on focus (only hover)

This is problematic (as explained in #327), and I suppose, a blocker for using native tooltips from <title> elements.

The good news is that Chromium is working on displaying tooltips from title attributes on focus (and SVG <title>). There's also a Firefox bug and WebKit bug.

I guess there's nothing else to do but to close this for now, may revisit in the future once there's some browser support for this.