Patial fix for #971 - prevent extents display being obscured by other status messages. The part not implemented yet but requested in the ticket is to be able to manually edit and set the extents. The implementation here provides a toggle button on the status bar to switch between display of mouse coordinates and current extents.

git-svn-id: http://svn.osgeo.org/qgis/trunk/qgis@9017 c8812cc2-4d05-0410-92ff-de0c093fc19c

Author: timlinux