Add tooltip on focus + hover

[ahmadayubi]

2021-03-01.11-26-56.mp4

The first square is created using:

<feature zoom="2" class="refDiff">
  <featurecaption>Accessible Square</featurecaption>
  <properties>
    <h1>Test</h1>
    <a href="www.example.com">test</a>
  </properties>
  <geometry cs="tilematrix">
    <polygon>
      <coordinates>11 11 12 11 12 12 11 12</coordinates>
    </polygon>
  </geometry>
</feature>

Closes #327

[prushforth]

I believe the <title> only is presented if it is found in inline layer DOM, not found in <layer- src=...> layers. Let's wait until we've figured out what this element will be called during upcoming meeting hopefully.

[prushforth]

OK, so per the discussion at today's meeting with @shepazu , we'll go with <featurecaption> for now.

[prushforth]

Darn I just noticed that when you use the ">" or "<" affordances you don't get the tooltip when focus goes to the next/previous feature. Sorry I should have checked more thoroughly.