Refactor Popup/Tooltip to use common code from DivOverlay

[johnd0e]

This is continuation of #6613.

(Each commit is self-sufficient and can be reviewed independently)

Edit: rebased on #7531.

• internal changes
• DivOverlay: new public api:

   // @method openOn(map: Map): this
   // Adds the overlay to the map.
   // Alternative to `map.openPopup(popup)`/`.openTooltip(tooltip)`.
  
   // @method close(): this
   // Closes the overlay.
   // Alternative to `map.closePopup(popup)`/`.closeTooltip(tooltip)`
   // and `layer.closePopup()`/`.closeTooltip()`.
  
   // @method toggle(layer?: Layer): this
   // Opens or closes the overlay bound to layer depending on its current state.
   // Argument may be omitted only for overlay bound to layer.
   // Alternative to `layer.togglePopup()`/`.toggleTooltip()`.
  

  Notes: openOn used to be Popup-only method, close was private method.

Todo: consider johnd0e/Leaflet@refactor-open-popup-tooltip...johnd0e:refactor-open-popup-tooltip-bind

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This change is 

[mourner]

👍

[johnd0e]

[...] or use Map.addLayer to open as many as you want.

This is not covered by tests.

Also: layer.togglePopup/Tooltip are not covered.

[mourner]

@johnd0e can you resolve the conflict in Tooltip.js?