Add FilterUnion node to combine the results of two or more filters

[bentoogood]

An example usage would be:

PathFilterA returns the paths to match charAssetA from a scene cache.
PathFilterB returns the paths to match charAssetB from a scene cache.

user wants to apply one operation to both characters. her current options are to:

• make another filter can duplicate the expressions/paths from PathFilterA & B
• use two duplicate nodes to apply the same operation (using one of the two original filters for each of the two nodes)

a workflow that might be preferable is to use a FilterUnion to combine the results of the two orig filters into one uber filter.

[johnhaddon]

We should also give some thought to a general purpose BooleanFilter - the thinking being whether or not we can support the AncestorMatch and DescendantMatch rules properly.