Add in-app way to change a config file

[ClementTsang]

See here for the milestone.

General overview

• This adds an in-application screen to manage the config file.
• This is going to be the "intended" way for a user to manage it now. A user can still change the config file manually if they want.
• Flags will still remain - the idea was always that flags were for usage "for that one run", and I'll keep that mentality.
  • (TBD) Likewise, flags will still override the in-application setting. The indicator for something set by the flag only will be different? Maybe a different colour or something.
• I plan on currently adding 4 sections to the config:
  • Display options (what is currently flags in the file)
  • Layout (managing widget layout)
  • Colours (managing colour choice)
  • Filters (for disk and temp)

Some potential problems

• I want to keep comments on what each setting means in the file... but I don't know if I can even do that. From looking at Serialize doc comments toml-rs/toml-rs#274, it might not be doable...?

[ClementTsang]

Closing this in favour of a milestone: https://github.com/ClementTsang/bottom/milestone/1

[ClementTsang]

Never mind, milestones look absolutely terrible, going to keep this issue open.