FindQt: fix setting DESIRED_QT_VERSION if "find_package(Qt VVV)" was called
In case the given version was not only "3" or "4", but something like "4.8" DESIRED_QT_VERSION was set to an unsupported value. While at it also check that the version passed in is really in the range of 3.x and 4.x. Also suggest switching to the more specific find modules if possible.
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