Fix bug in detecting text in MenuSearch

A bug sneaked into my implementation of Boyer-Moore-Horspool. This lead
to not finding certain patterns. Given the text 'abcdde' and the pattern
'dd', the faulty implementation would not find 'dd':

1. 'ab' does not match, skip 2 (length of pattern)
2. 'cd' does not match, skip 2 (length of pattern) <- the bug.
3. 'de' does not match, end of string

The bug in step 2 is to not use 'd' to detect how far to skip but to
use 'c' (which is not in the skip-table) and thus 2 bytes are skipped).

src/FbTk/MenuSearch.cc

@@ -75,7 +75,7 @@ size_t search_str_bmh(const std::string& text, const std::string& pattern) {
                 return t+p;
             }
         }
-        t += skip[std::tolower(text[t+p])];
+        t += skip[std::tolower(text[t+plast])];
     }
 
     return std::string::npos;