applets/brightness: fix Return press on switch

The "Suspend" switch for Night Light calls toggle() on space/enter/ return press, and handles the onToggled() signal to actually perform the action. The qqc2 api is exceptionally confusing here, toggle() refers to changing the checked state of a button, but does not automatically call toggled() which is called when a user interactively manipulates a control, and would need to be called explicitly in the handler. The only reason it worked for Space is that the event wasn't accepted, so the switch called toggled() internally.

Instead, just use clicked() and avoid the whole messy toggled/toggle() situation. Along the way, we also explicitly accept the onPressed events when we handle them, to avoid problems with multiple activation in the future.

BUG: 500702 FIXED-IN: 6.3.2