do not allow multiple identical wms layers

[richlv]

build 1566.
some time ago, wms plugin did not create a persistent layer - instead, choosing desired wms from the menu downloaded visible area. now, choosing wms source again, adds identical layer.

this should be prevented, preferably, with a popup message informing user.

now, if there's some use case where this is actually desired, added wms layers should have different name (like "Landsat 2", "Landsat 3" etc), and every identical wms layer past the first should default to "automatic downloading" off.

[skyper]

Imagery should also prevent to open the same layer (+ same settings) twice and name the layers properly not just the provider.

[Don-vip]

In 5458/josm:

fix #2519 - do not allow multiple identical imagery layers

[skyper]

Now it is impossible to have two layers of the same provider but with different shapen values set. I often use the same provider without shapening and with smooth shapening. The major disadvantage was that you did not see any difference in the layer list and had to aktivate them to know which one was with option.

As I already commented, please allow several layers of one provider but with different settings and chose adequate names for the layers.

Thanks.