COMP: fix qt warnings and c++11 errors Plot GUI
fix as reported by @lassoan https://github.com/Slicer/Slicer/pull/797#issuecomment-333815222 tested on ubuntu 17.10 gcc6.4 qt5/VTK8 qt4/VTK8 (so this fix is not really tested. it needs to be tested on the build with VTK7).
if the fix can wait I'll try to test qt4/VTK7 version on mac El capitan