Buttons have a "down" property. IIRC you can set a onDownChanged event handler. If down == true, set an interval using javascript's setInterval, in which you increase/decrease the brightness every x milliseconds. Clear the interval when down gets false.

The documentation refers to MouseArea, which give you the "pressed" property, and thus onPressed and onReleased. I'm not sure whether Button inherits these tho.

Incidentally, Qt 5.9 introduced "pressAndHold"...