Fix the Plasma::Applet::Actions crash

We were mixing KScreen and QScreen API.

Corona.cpp checks we are requesting a containment for a valid screen
if (screen >= 0 && screen < numScreens()) {

This fails as numScreens() is Qt API based, whereas the signal we're
adding the output for is ShellCorona::addOutput

BUG: 351777
REVIEW: 126961