Proposal: Change order of data layers

[sickel]

A heat map on top of a point layer may cover points completely. If one ends up in that situation, the only way to recover is (as far as I have seen) to delete both layers and recreate them, this time with the heat layer created first so it goes "to the bottom". It should be possible to drag one layer to a specific position, cf the handling of layer in any GIS software, eg QGIS.

If this for some reason should be difficult, if a heat map is always plotted below any point layer, this will probably solve most of the cases. (In GIS one would hardly ever put a raster layer, like a heatmap, on top of a point layer)

Also cf #103