Icons for layer types in the layers list

[SylvainCorlay]

It would be really cool to have a consistency between the icons used for layers in the layers list and the types of layer available in the toolbar.

[gjmooney]

One thing to think about here is that the toolbar commands are convenience options, so they add a source AND a layer at the same time, but there isn't a 1-to-1 link between source and layer types. For example a shapefile source and geoJSON source both use a vector layer.

[brichet]

I wonder if we couldn't have a symbolism that refers to both the layer type and the source type.