Updated Qwt to 6.1 as it supports Qt5 and Qt4

Functionality seems to be identical as far as I can tell, though I'm not as comfortable with the plotting code as I am with other areas of the codebase, so some extra eyes on this would be appreciated.

One note, the PrintFilter code has been commented out with no correction as it's unclear how it should be fixed.

I've been exploring more of the future Qt5 transition, and it looks like we'll be blocking on QtCreator's util library, as that's still Qt4, though we could patch it in our tree if we really wanted to. 

With this and #722, there aren't that many required changes to port everything to Qt5.