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.