Depth widget ideas (simple to fancy)

Transferring from [our internal discussion](https://bluerobotics.slack.com/archives/C03FLG1LVUM/p1669933540534559) 3 months ago / [the initial PR](https://github.com/bluerobotics/cockpit/pull/141#issuecomment-1354957266):

- extents should show the surface and an optional water column depth
- if no altitude sensor either allow setting a "known bottom distance" variable, or just have no explicit bottom shown
- local crop -> show zoomed in local area
- dots (bubbles?) above and below to show that some distance is "missing"
- number of dots gives a sense of relative distance (e.g. if close-ish to the bottom but far from the top, maybe have 4 dots at the top and 1 at the bottom)
- zero dots when an extent is within the 'zoom' region
- trail should fade in from left to right, with configurable widget width and shown duration
- time scale should be linear by default (-> constant velocity shows a straight line)
- may be worth providing a logarithmic timescale option, to allow compressing more samples history into the available space
- altitude sensor readings should also be trailed, if they're available
- display of current position and altitude would be cute to show as an ROV (with tether trail) and an anchor
- ideally (long term?) the vehicle icon would be configurable, so people can put their own design instead
- maybe show the surface as wavy water 〰️ 🌊
- could show vehicle pitch (per #1053)

Start by reviewing the initial PR #141 and the pitch for vehicle pitch in #1053, then compare those discussions with the depth-widget ideas listed here. The issue has no named files, entry point, tests, or single acceptance criterion; a concrete scope and definition of done are needed before implementation can begin.

Written by the indexing model from the issue text.