Better visual representations for dependencies

[levinli303]

add support for showing dependencies of add-ons, maybe grey out install button if any of the dependencies is missing.

[DaveBowman2001]

I don't know why I only noticed this very late, but this would be definitely a very useful feature. For instance, most of the visiting spacecraft (Soyuz, Dragon, etc.) are separate add-ons from space stations (ISS, Tiangong, Salyut, etc.)

[levinli303]

you can specify dependencies in https://celestia.mobi/submit-addon now. however I saw that in the existing description of add-ons. Space Shuttle depends on ISS, and ISS also depends on Space Shuttle. Dependency cycles should be avoided.

[DaveBowman2001]

Dependency cycles should be avoided.

@levinli303
It's a little bit complicated due to the space exploration history behind it. The space stations can definitely work without the required add-ons, but the latter won't work at all without them (i.e. the Soyuz, Progress, Shenzhou, etc. all go to space stations)