bulma (call): add a button for screen sharing: - add a button for screen sharing, only on desktop - split `muted` state to `inactive` and `muted` where `muted` is only about the color - use the `inactive` state with an other color (in `screen-off` class) to indicate when screen is not shared rel 432

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    <span class="has-text-info is-size-7">{% trans %}new messages{% endtrans %}</span>
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