layout-manager: Handle cutouts in the indicator areas

Shift elements towards the center unless that would result in massive movement.

For smaller cutouts within the indicator boxes we'll insert stub elements at some point in the future.

In order to make that feasible we switch from using box margin to CSS as that allows us to identify first/last children independent from whether this is a clock or indicator box.

Needs likely more testing but should hopefully be good for an initial review round.

For testing gmobile!59 (merged) can be used.

Demo: https://ruhr.social/@agx/114054228517776080

Note that we don't make any arrangement to avoid overlapping with the lock / power off button in the unfolded top panel yet, that needs some design upfront as it's not clear where to best move them. One possibility would be to move them downwards like we do for the clock.