Add ITextMark::visible() to make ITextMarks not take any space

Change-Id: Ib07cdc5cbe9a26385f82e6a845d72f218de7b2a6
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>

src/plugins/texteditor/itexteditor.cpp

@@ -72,6 +72,10 @@ ITextMark::Priority ITextMark::priority() const
     return m_priority;
 }
 
+bool ITextMark::visible() const
+{
+    return true;
+}
 
 double ITextMark::widthFactor() const
 {