[BITV] Check color picker for accessibility requirements and fix all found issues

[JuliaKirschenheuter]

We need to make sure that our color picker is fully accessible. Small list of requirements:

• contrasts
• keyboard navigation
• all needed aria attributes for each color button and action button with right naming

If there would be too much issues, an alternative could be to use a native color picker instead.

Current state:

• Focus state on 'change schema' button is not there:
  
• description for all buttons is not there

[ShGKme]

Working on it. I'm going to do 2 things:

• Replace vue-chroma package with a native color picker. It works in all modern browsers, accessible and advanced. ( [NcColorPicker] Get rid of vue-color in flavor of native picker for custom color #6096 )
• Add the optional ability to add a name of the color in the palette (but with l11n...)