Change the behaviour of chatmarkdownhelper to only work when a single character is added.

This is because if pasting a link for example * or _ could be treated as formatting. We now assume that any pasted text is done as plain text and is wanted as is.

src/libneochat/chatmarkdownhelper.h

@@ -42,6 +42,6 @@ private:
 
     void checkMarkdown(int position, int charsRemoved, int charsAdded);
     void updatePosition(int position);
-    void checkMarkdownForward(int charsAdded);
+    void checkMarkdownForward();
     void complete();
 };