Move UI bits to a libcamera-ui
In order to take photo without unlocking the phone (https://gitlab.gnome.org/World/Phosh/phosh/-/issues/678) it would be great if the ui bits could be moved to a library that can be embedded e.g. by phosh so there's no duplication and the camera application and the shell can use the same ui without having to keep things in sync manually.
We're doing this for phone call ui (https://gitlab.gnome.org/World/Phosh/libcall-ui/) and that proved to be useful. In contrast to what we do with phone calls we likely don't want to invoke millipixels
(or the final camera app) via DBus but likely set up the processing pipeline on the phosh side so it would be great if that would be easy via a library too so there's no feature drift.
(I'm keeping this deliberately vague since I don't expect that to happen soon and likely not in millipixels at all but wanted to file an issue to express what we'll need further down the road once photo taking becomes more automatic (and we're moving closer to a non-tech preview camera app). I think libaperture goes into this direction (https://gitlab.gnome.org/jwestman/libaperture) already for GTK3.