replace list of snapping markers with a single marker

In some cases during digitizing, there are several snapping markers drawn: to reproduce, activate snapping with a rather large tolerance, enable snapping on intersection then add a new feature near an area where several features already are. The result is something like this: 
![2014-02-14 18_54_19-qgis 57dd877 - project](https://f.cloud.github.com/assets/537624/2170167/6d15b2de-9567-11e3-90dd-b6c382c7c03c.png)

This seems like a bug to me, very confusing to the user, but the code was written explicitely to support this (by @jef-n in b8642a93faac8cb30b5dd43200ef1c5797175bfd), so I wonder if there is a use case I didn't think about?

This patch replaces the list of markers by a single marker.