Move to Qt 5.12 for ES7

[kbumsik]

Qt 5.12 will be released around December. I'm going to use this as soon as it is released because Qt 5.12 adds ES7 support in QML! I will rewrite my QML code to ES7.

Other than ES7, I will examine if there is any other great features I can use for VirtScreen, especially Quick Controls 2.

[xosg]

Yeah, we r so excited to welcome ES6/ES7 in QML !