use qt4 as default and if it is not found use qt5

These CMake files look for QT4 in the first place and Qt5 in the second place. I tested it on Archlinux and Ubuntu using the following scenarios:
- Qt5 installed -> works
- Qt4 installed -> works
- Qt4 and Qt5 installed -> works Qt4 is used
- no Qt installed -> cmake gives an error

The reason why I decided to look for Qt4 in the first place is that older versions of CMake do not necesseraliy come with a find scripts for Qt5. This would result in an error.