Support Qt5

[opoplawski]

We'd like to move VTK to Qt5 in Fedora, but engrid seems to require Qt4:

CMake Error at /usr/share/cmake/Modules/FindQt4.cmake:1318 (message):
  Found unsuitable Qt version "" from NOTFOUND, this code requires Qt 4.x

[ftpronk]

Updating our software to compile against Qt5 is high on our priority list, but, unfortunately, we do not have the necessary resources available at this time to look into it..

Although the Qt changes will probably be minor, we would also like to to update to VTK 7 (or 8?), and that might bring up a few unexpected issues.

[opoplawski]

FWIW - Fedora is already building engrid against VTK7.