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