Styling/theming of Select and SelectMenu

[hastebrot]

The icon of Select components can't be changed.

The Select component of a SelectMenu ignores theme configuration.

[hastebrot]

My plan is to allow replacement of <Icon /> via a new prop.

// Select.tsx
{isLoading ? <LoadingSpinner color="text" /> : <Icon />}

Maybe there should also be an icon property in the Select theme options.

Same applied to SelectMenu:

// SelectMenu.tsx
{isLoading ? <SelectMenuLoadingSpinner color="text" /> : <SelectIcon />}

Additionally SelectMenu should be themed using SelectMenu.Select theme option or Select theme option.