[CMake] FindQt.cmake for Qt4

[John Biddiscombe]

>>Do we have version specific Find*.cmake files for anything else?
>>
Why not
FindQt(...)

Inside FindQt, it does a FindQt3, FindQt4, FindQt5 etc, then if it finds 
multiple Qt versions installed, it provides a new GUI option
Set(QTCOMPATIBILITY cache stuff etc 3 or 4 or 5) the include paths can 
then be setup by the find qt module depending on which was found (or 
overridded by the user in the gui), by default it will use the highest 
version available, but the user can force 3 comptibility if desired.

JB