Formatted call to filter_documents().

This solution seems to be the least painful one: filter_documents is called
with an 'union' filter object and it's member 'union' is simply a list of
documents with which the union is composed. An empty list would leave the
documents as they where.
Bypassed union-ized documents from filtering.
Switched to upper-case variable name.

The filter form wasn't doing anything anymore, and I didn't even touch the
keep-selection button. For some reason the js engine gets confused with my
choice of variable names, so I worked around it thusly.
Implemented union of two document sets.

The 'cancel' button still does nothing, but I like how dynamic objects saved
the day ... AGAIN: If the document is an element of filters[0].union, it is
tagged as such. The tag is removed after it has been used.