Markdown: Use different background color for code

Sets the background color for code blocks and for inline code (actually
for any fixed font text).

Change-Id: I1365111b38314a8e0a129aca35d754ad3f2f13bb
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/libs/utils/stringutils.h

@@ -130,7 +130,10 @@ public:
     void highlightBlock(const QString &text);
 
 private:
+    QBrush codeBgBrush();
+
     QBrush h2Brush;
+    QBrush m_codeBgBrush;
 };
 
 } // namespace Utils