HPCC-9638 Filter "Clear" does not reset some drop down controls

Fixed TargetSelectWidget to detect "null".
Re-factored Select controls into TargetSelectWidget.

Fixes HPCC-9638

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #4611 from GordonSmith/HPCC-9638

HPCC-9638 Filter "Clear" does not reset some drop down controls

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>