ENH: Fixed loading of FreeSurfer overlays with custom extensions

Loading of FreeSurfer scalar overlays from files were offered on the GUI for any file extensions,
but an internal check prevented loading from files with non-standard file extension (for example,
loading from lh.thickness worked but loading from lh.meanthickness_contrl did not).

Solved by assuming that overlays with custom file extensions can be read by vtkFSSurfaceScalarReader.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26068 3bd1e089-480b-0410-8dfb-8563597acbee