Elliminate flickering when calling mergeNewParams

We are calling mergeNewParams quite alot, to make small incremental changes to our tileset. The constant flickering is driving us mad. I  would guess that we are not alone in wanting a more fluent behaviour of mergeNewParams.  The following patch implements this, by adding a reload method to HTTPRequest layers, this is then specialised in the GridLayer where replace is called on all grid tiles.
By default "replace" on a tile, will just redraw it, while on an Image tile, it will delay the redraw until the new image has been reloaded, which will then dom replace node the old image.