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.
it would be good to keep this list outside the `if()` block so that we don't have to repeat it twice (which could lead to someone adding an example to only one and not the other). we could also do the same thing with the qimage_blur example and just make a list with a single item called `QT_EXAMPLES`. then the Qt4 and Qt5 side of the loop can just iterate over each list and set them up as necessary.