import filters accept broken syntax

[westram]

1. accepts missing closing quote:

   WRITE "cell_type
   

   See rdp.ift​.
   ⇒ internally fieldname is "cell_type i.e. includes a quote and is invalid
   (see also 'list of available fields' in FTS edit window when rdp.ift is selected)
   TODO raise error (while loading filter!) + fix .ift(s)

2. accepts unescaped quotes in ACI

   SEQUENCEACI     "remove("0123456789 /")"
   

   Also see rdp.ift​.
   TODO
   • Test whether that ACI works at all.
   • Quotes should be escaped ⇒ raise error while loading filter + fix .ift(s)