Fix flag colours and marked/suspended/buried colours not showing up

[iamllama]

Closes #38

This fixes flag and marked/suspended/buried colours not showing up, by specially handling those cases and using ARGB hex instead of rgba(...)

[BlueGreenMagick]

Feels a bit hacky, but if it works, it works.

I think it'd be better to change hex_with_alpha_to_argb() function to check if hex_color.startswith("#") and len(hex_color) == 9:. Colors written as svg keywords (e.g. lightblue) should continue to work.

After the change, one downside is that colors written using argb(x, x, x, x) syntax will no longer work. But it's probably fine since I don't think anyone uses it anyway.

[iamllama]

Added the mentioned check and support for rgba(...)

After the change, one downside is that colors written using argb(x, x, x, x) syntax will no longer work

Not sure what you mean by this. It never even worked to begin with, since QColor doesn't accept it and it isn't even part of the CSS standard, nor sass (unless you originally meant rgba(...))

[BlueGreenMagick]

LGTM!

I don't have access to its Ankiweb account, so you'll have to wait. I'm not sure who has it currently.

[iamllama]

ACCENT_CARD and ACCENT_NOTE also need to be included in ARGB_ONLY_ENTRIES too, unfortunately, but they're blocked by ankitects/anki#3629