Add support for l-pane custom element

[andrewgryan]

Adds <l-pane> HTML custom element that can be used to support Leaflet JS panes.

E.g. the proposed CSS and HTML in a full app will look like this:

CSS

/** Control z-index within Leaflet standard panes */
.leaflet-foo-pane {
   z-index: 650;
   pointer-events: none;
}

HTML

<l-map zoom="1" center="[45,15]">
    <l-tile-layer ...></l-tile-layer>
    <l-pane name="foo">
         <l-image-overlay>...</l-image-overlay>
    </l-pane>
</l-map>

[andrewgryan]

This establishes a nice pattern for custom panes and a test suite to make it easy to support all elements in the future