Views filter: Weight enabled

Issue Summary

I'd like to propose an addition to the views integration of the weight module. It would be nice to filter by node types that are weight-enabled. I think it shouldn't be too hard to chech if a node's type is in the weight_node_types variable, but I'm not a views expert unfortunately.

With a weight-enabled filter, one could easily create a universal interface to sort all weight-enabled nodes. I have such a view now: The first argument is tied to the node type and if no argument is present, the user sees a summary of weight-enabled node types. This way, the user first selects the node type he wants to sort before he gets to the weight changer table. Currently, I hand-picked the weight-enabled node types in the node type filter. That filter has to be changed manually when another node type is weight enabled, where a weight-enabled filter would automatically adapt.

I hope my non-native English was sufficient to get the idea across.