User-provided color scheme

[cassidyjames]

We've been getting a handful of PRs to add one-off color schemes, but these are highly subjective and personal. Instead, we've talked about adding a way for a user to set their own color scheme.

[danirabbit]

Prior Art

Tilix:

Xfce Terminal:

GNOME Terminal:

[danirabbit]

It doesn't seem like any of the prior art is particularly contextual and it's hard to see how it will relate to the end result. Off the top of my head I feel like a good solution:

• Doesn't require hunting around in a preferences dialog. We can add a ( + ) add the end of the list of color options in the popover we have currently. This could then throw a dialog that specifically and only manages the stored color schemes.
• Includes a preview so we can see what the resulting color scheme actually looks like
• Labels the color selectors (of which it looks like only Tilix tries to do this)
• If we're really smart, we can do some color contrast checking and warn about color schemes that would be illegible. At the very least we should do this between the background and foreground color and make sure it passes the base A level