Allow selector to be used with fixed width

From TEIID-2898: To allow for fixed width files containing multiple record types, the selector functionality should apply to fixed width as well. At least initially this will be restricted to only the overall selector as positional selector column references won't be possible. See the reference for more: https://docs.jboss.org/author/display/TEIID/FROM+Clause#FROMClause-TEXTTABLE

Also from TEIID-2896 fixed width parsing will now make an effort to parse lines that are too short, rather than immediately throwing an exception. If there is designer logic with that limitation, it should be lifted.