Sqlite: Improve SqliteStatement

There are now only value and values methods. value returns an optional
and values are returning a vector. The result value count has now be
specified instead of the result value list.

Change-Id: I17a0741d5e838b4bf4b9486825c870ada1722584
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/unittest/sqliteteststatement.h

@@ -46,7 +46,7 @@ public:
     using Statement::execute;
     using Statement::next;
     using Statement::text;
-    using Statement::value;
+    using Statement::fetchValue;
 
 protected:
     void checkIsWritableStatement();