fix for gxp #191 - allow automatic wildcard attachment in LIKE Filters

See GXP issue #191. Already fixed some time ago and confirm working at several sites. Appearantly I forgot to make a Pull request. Together with issue #189 (QueryPanel: allow case-insensitive string comparison WFS Filters) which has been already merged, automatic wildcard attachment in the case of LIKE Filters is very useful, as this is user-intuitive. See an example here:

http://lib.heron-mc.org/heron/latest/examples/querybuilder and try 
STATE_NAME LIKE al
(Alabama+California). Both case insensitive match and wildcarding is applied. Both are configurable (default is false).
GeoExt had in addition the option to either pre and/or postpend wildcards, but this I think is not really required for most uses.