Sensors logging: improve control on logging and subtitle overlays

[ES-Alexander]

Continuing on from #450, there are various improvements we could make to the current subtitle output format, and the configuration process.

As some additional context, there are future plans to allow replaying a video file + Cockpit log through Cockpit itself (see #619), which should have considerably more configuration and nicer interface elements than subtitles can ever hope for. We should still try to make the subtitles nice for people who need to use them, but we can't dedicate too many resources to them because that would just end up redundantly replicating Cockpit's interface in a different format.

That said, here are some improvement ideas for the current logging and subtitle system:

• allow controlling the logging rate (currently hardcoded to 1Hz)
• support including one or more vector-based logos
  • ideally this would include a company logo, along with optional checkboxes for Cockpit and firmware logos
  • we should support users inputting an SVG which we convert for them into the Advanced SubStation-Alpha vector format drawing commands
    • this auto-converter may be handy, although we may also want to roll our own
    • we at least need to also support scaling and some form of position offset
  • It could also be valuable to make/recommend a raster -> vector image converter that's focused on logos, since that's what we're expecting people to want to display, and users may not have a nice vector source already.
    • this could be hard to do, although could also be really useful as a "public good" thing if we made it a web-app or something
• allow widgets to self-register for logging
  • this would require defining an API for them to do so
    • Support playback from Cockpit log files + video recordings #619 discusses a related idea for playback purposes -> they should likely be co-developed
  • it would be necessary to refactor the current widgets to use that API
  • some additional relevant discussion
• provide a configurable interface for the output file, based on what the subtitle format supports
  • the current logged-variable checkboxes can be turned into a list of draggable elements
    • vector logos and the mission title / local date+time would be additional elements
  • we can provide a 9-cell grid that elements can be dragged into and ordered within
    • I don't think we need to go too complex with this - we just need some basic location and ordering controls
    • we can make use of the alignment specifiers
      • I recommend the \an 1-9 numpad-style ones, because they're less confusing than the standard \a ones
      • if we do this but still want left-aligned text in the columns then we may need to right-pad each value in a column (with spaces) to the width of the widest row in the column, and may also need to require a mono-spaced font to do so effectively
  • should allow setting a default fontsize, and text/border colouring
    • could allow setting default font, although we may just want to decide on one
    • setting individual font sizes and bold/italics and colours is possible, but is likely more effort than it's worth
  • being implemented in Improve subtitle telemetry display and configurations #946

[rafaellehmkuhl]

That one will be VERY cool.
Adding the columns with draggable functionality is actually not that hard, and it's a good start.

[rafaellehmkuhl]

Bjarte has suggested in our forums to take a look at this solution for inspiration.