[applet] Enable KScreen applet by default

This applet is quite handy to quickly change screen distribution, presentation mode or quickly access the complete kscreen settings.

Being disabled by default makes it almost undiscoverable for most users.

I think this can reduce the many (legit) requests about "quickly access to screen settings", but on a more proper place.

@teams/vdg as it adds an applet to the default systemtray. It automatically shows when there are more than one screen.