make components extendible (e.g. tm_geocoder)

Just added `tm_geocoder()` to `tmap.mapgl`.  The new vignette: https://r-tmap.github.io/tmap.mapgl/articles/components

However, it fails in plot and view mode, because this does not work:

```r
#' @export
#' @method tmapGridCompHeight tm_geocoder
tmapGridCompHeight.tm_geocoder = function(comp, o) {
	comp
}
```  

`tmapGridCompHeight` is not exported.
