Get incoming calls UI working on master/Qt6, fixes #74

Marked as draft as there is still a lot to be fixed...

About the QtGraphicalEffects topic (source):

The MultiEffect type, the successor to the deprecated Qt Graphical Effects from Qt 5, applies a post-processing effect to the source item.

• plasma-dialer/src/qml/call/ActiveCallView.qml wasn't even using QtGraphicalEffects so I've just dropped the import line
• plasma-dialer/src/qml/call/Avatar.qml , I've tried to put together something reasonable based on Qt documentation, but haven't been able to test because the "Accept call" button does not work

(partial?) list of what remains to be fixed:

• incoming call (accept/reject buttons) screen does not appear automatically: from the other phone you hear the ring, but the incoming call screen does not appear until you open plasma-dialer (Update: to be investigated, maybe it appears for the first incoming call, then something breaks and it does not appear until app gets manually opened)
• "accept call" button does not work (or for whatever reason it does not switch to the active call screen)
• accept call when device is unlocked (from the notification)