Styling of optgroups in dropdowns

[eomine]

As discussed in recent meetings, I changed the optgroups in dropdowns so that the options are not indented (replacing optgroup tags with disabled option tags).
But seeing the result, I think the previous version with the indented options was clearer (see comparison below).
A better solution would be keeping the indents and changing the optgroups labels to have the same color/opacity than the actual options. But this is not feasible unless we use appearance: base-select, which is currently an Chrome-only feature with some possibly significant drawbacks.
I think we should keep using native selects for now, while considering using a 3rd-party component in the future.

Before	After

[graphicore]

When it's just currently chrome only it's not that bad. Chrome is our most important target and when it's going to be caught up by the other browsers it's even better. Maybe you can use it to improve the most pressing issues with the minimal possible effort using appearance: base-select. The spec of it might be changing, so I'd keep it simple.

[eomine]

Ok, I'll work on it then.