Add group search

* Allow searching by group using the `group:` field.
* Group hierarchies can be searched by including a '/' in the search term.

tests/TestEntrySearcher.h

@@ -36,6 +36,7 @@ private slots:
     void testAllAttributesAreSearched();
     void testSearchTermParser();
     void testCustomAttributesAreSearched();
+    void testGroup();
 
 private:
     Group* m_rootGroup;