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/debugger/debuggerengine.cpp

@@ -156,6 +156,7 @@ LocationMark::LocationMark(DebuggerEngine *engine, const FilePath &file, int lin
     : TextMark(file, line, Constants::TEXT_MARK_CATEGORY_LOCATION), m_engine(engine)
 {
     setPriority(TextMark::HighPriority);
+    setIsLocationMarker(true);
     updateIcon();
 }