More verbose GTK Window Scaling label.

Hi!

A friend started his journey with Linux, using a XFCE-based DE and a hi-res display. Wanting the UI of various apps to scale, he changed the Window Scaling option to 2x, however it only affected GTK-based apps. I find it impossible to blame him for believing it would also affect other toolkit-based apps (QT) since the label doesn't specify it's GTK only, and so I added a clarification.

I would also recognize as a potential solution if the dialog set the env var QT_SCALE_FACTOR to the same multiplier as it does for GDK, but it would be a bigger change, and probably harder to push through than the label change