Change image storage to use new ExtendedAttrs payload field

[PerBothner]

Adds a general-purpose payload hook to ExtendedAttrs. This allows us to get rid of the ExtendedAttrsImage logic, which can easily conflict with other extensions.

The PR avoids looking into BufferLine internals by instead using loadCell and setCell. The calls to loadCell may add slightly more overhead; if that is measurable, we can look into reducing the number of loadCell calls.

This is an implementation of this suggestion.

It might make sense for a future update to remove the urlId field from ExtendedAttrs and instead use the general payload hook.

[PerBothner]

An optional optimization is to replace imageId (which needs to be looked up in the _images map) with a direct reference to IImageSpec:

class ImageTileInfo {
  constructor(
    public imageSpec: IImageSpec | undefined = undefined,
    public tileId = -1) {
  }
}

A further possibility would be to remove the _images map. Since each IImageSpec has an associated Marker, when we need to iterate over all images, we can do that by iterating over markers. This assumes we add a payload field to Marker (as suggested in PR #5853), and that the payload field is used to point back to the IImageSpec.