In Qt Creator 3.0.0, when including the file <QString>, or any other header file in the Qt Widgets library, there is a dark green squiggly line below it (i.e., an error), which says:
QString: No such file or directory
I am also having some Compiler issues:
cannot find -lGL
collect2: error: ld returned 1 exit status
I do not know if the Compiler errors and the <QString> file problem are related, but if they are, I will report it here.
Google the errors and see if you find any solutions. If you don't find a solution, then come back here or got to a Qt specific forum to find help. I did a google search and found several results for that error.
As for GL, are you sure you have the Open GL library installed on your computer? If so, are the files included in your compiler and linker search path?