#686 Bump Qt4 to Qt5

[Kor de Jong]

It won't be long until Qt4 is deprecated on Linux distributions.

• http://perezmeyer.blogspot.de/2014/11/early-announce-qt4-removal-in-jessie1.html

[Oliver Schmitz]

5.6 a good option?
http://blog.qt.io/blog/2015/12/18/introducing-long-term-support/