End support for QT4

[splintermind]

• [ ] Move the current code to an archived QT4 branch and end support
• [ ] Remove the #if QT_VERSION conditionals everywhere in the current main branch
• [ ] Update the .pro to build only QT5
• [ ] Update the travis.yml to exclude QT4

QT5 was released December 19, 2012 and maintaining backwards compatibility is becoming both cumbersome and time consuming and will only get worse. eg: regular expressions, json, qnetworkreply etc.

[Hello71]

in the process it would be nice to also migrate to CMake.

[jecowa]

Will this have any effect on Mac users? Will a QT5 version of Dwarf Therapist still run on Mac OS X 10.5 Leopard? (I ask about this version because Toady builds Dwarf Fortress on Leopard.) Thanks!