Fix validation of PathInputPanel

It should be distinguished between validation of the chosen path and notifying the user of an invalid selection.

Previously, user would be notified of invalid paths even during panel initialization phase, when path validation is used to figure out the best available default value. This was a problem with the JDKPathPanel, which calls pathIsValid() several times in an attempt to find a valid default.

Also fixed incorrect documentation of where a processor XML tag is supposed to be placed inside an (input) field element.