improve escaping characters from multibyte strings

Read the whole character from a multibyte string and then escape it.

while here:
- add more virtual methods for escaping strings to BaseExpression

src/dbexpression.h

@@ -37,7 +37,6 @@
 
 #include <vector>
 #include "baseexpression.h"
-#include "morm_types.h"
 
 
 namespace morm