Well well well... It works...partially :p
Firstly, I got a...warning (in french) :
"The QML module contains some C++ modules, reading type informations...".
So, GridMenu is recognized by the code model, but if I run the program it doesn't work anymore :
"QQmlApplicationEngine failed to load component
qrc:///main.qml:5 "../libs/customquick/controls": no such directory"
Maybe because main.qml is within a Qt resource file ?
EDIT : ok...it's because the qrc file. If I copy main.qml in the build directory and I delete it from the resource file, It works. The "import" is still yellow underlined.