Add option to control layer insertion position in addLayer action

[manisandro]

Not sure if this is the proper approach, but I didn't find any way to control the position a layer is added. By default it seems new layers are always added behind all non-background layers, but we need a way to add the new layer on top.
If this approach is correct I'll add tests.

[mbarto]

Please, add a unit test

[manisandro]

So this means that the approach is ok?

[coveralls]

Coverage increased (+0.1%) to 80.903% when pulling d2534e0 on sourcepole:addlayerpos into 3c17884 on geosolutions-it:master.

[mbarto]

The standard behaviour is to have layers ordered from bottom to top in the order they are listed in state.layers.flat. This order is created in LayersUtils.sortLayers by browsing the TOC so that layers that are at the bottom of the TOC are also at the bottom of the map stacking order. So, in theory, your approach should work.

[coveralls]

Coverage increased (+0.1%) to 80.903% when pulling c024ad9 on sourcepole:addlayerpos into 3c17884 on geosolutions-it:master.