Add standard ExpandDecider subclass to restrict to terms with a particular prefix

Hi, I have done this patch for ticket #467, please take a look
1. Add class ExpandDeciderFilterPrefix(expanddecider.h/cc) and related testcase(    api_anydb.cc api_nodb.cc)
2. Add line 'generated-csharp/ExpandDeciderFilterPrefix.cs' and 'org/xapian/ExpandDeciderFilterPrefix.java' in related Makefile.am(Java and C#)