ftcustomfilter: Allow override of default label filter in ftjoin based library pickers
While you can override the ftlibrarydata for an ftjoin based formtool, you cannot readily change the way the results are filtered. The current behaviour is a like pattern match on label.
Improvement would be to:
add an ftcustomfilter=true/false metadata item
add arguments.filter to the ftlibrarydata invocation so you can see the filter
disable the default filter query