Swipeable notification frames

This takes the notification frame related bits from !905 so they become swipable in the content area and the lockscreen.

The only thing that might look odd is that we swipe frames and not individual notifications so e.g. if chatty has several SMS notifications we'd swipe a way all of them at once but I think it would still be a start and we could make individual notfificatinos swipeable as a follow up.

/cc @exalm since 100% of the code is their work

Closes: #666 (closed), #693 (closed)