lockscreen: Add a notification to go back to ongoing calls

Guido Günther requested to merge guidog/phosh:call-states2 into main

While one could swipe back to an ongoing call it wasn't clear on the lock screen that a call is ongoing. Add a notification for that. We do that in a way that will make it simple to have parallel calls once GNOME calls supports it.

I'll fix up the indent in the ui file once !1280 (merged) is merged

Demo: https://social.librem.one/@agx/110606029148580885

Closes: #591 (closed), #890 (closed)

Edited by Guido Günther

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