QTextImageFormat: add Qt7 TODO to remove the default value

Change-Id: I8b60b302c9df7ce7c44b89fe45f813c4532b7bd6
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/gui/text/qtextformat.h

@@ -832,6 +832,7 @@ public:
     inline qreal height() const
     { return doubleProperty(ImageHeight); }
 
+    // TODO Qt7: remove default value; setQuality() with no value reads strange.
     inline void setQuality(int quality = 100);
     inline int quality() const
     { return intProperty(ImageQuality); }