ktp-text-ui requires Qt 5.7 but does not at the CMake level

[Albert Astals Cid]

Niels,
  your "Port to QWebEngine" commit increased the requirement of ktp-text-ui to Qt 5.7 and now it does not compile.

https://build.kde.org/job/ktp-text-ui%20Applications-17.04%20stable-kf5-qt5/PLATFORM=Linux,compiler=gcc/4/console

You have two options:
 * Introduce some #ifdefs similar to what https://cgit.kde.org/kde-baseapps.git/commit/?id=bfd09bcec03e8080bfce33f850c54bcff3a4a475 provides.
 * Actually require Qt 5.7 in the CMakeLists.txt

The Beta tagging is in 25h, so please fix it ASAP.

Cheers,
  Albert