buildOptions was passing default values for absent attributes,
including empty-string defaults like pane:'', which caused Leaflet to
look up this._panes[''] and return undefined, crashing in appendChild.
- Skip empty-string defaults in buildOptions so Leaflet uses its own
- Broaden createChildRegisterHandler param from LayerGroup to Layer so
non-container layers (marker, circle, etc.) can intercept popup/tooltip
children and call bindPopup/bindTooltip instead of bubbling to the map
- Add child-registration handler to every layer component (all 12)
- Fix disconnectedCallback in container components to unbind popups and
tooltips and remove child layers properly