Editor: paint location marker over everything else

The location marker is updated very regularly and especially while
stepping moves one line at a time. Resizing this marker when it enters a
line that already contains a marker is visually distracting. To prevent
this paint the marker over all other markers in that line.

Change-Id: I63ad72384e77eeae4dc6d2e2c2ac77c88da92c56
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/texteditor/textdocumentlayout.h

@@ -203,6 +203,7 @@ public:
 
     int lastSaveRevision = 0;
     bool hasMarks = false;
+    bool hasLocationMarker = false;
     int m_requiredWidth = 0;
 
     void setRequiredWidth(int width);