Quick Editor: Alt Text - Improve behaviour on the Alt text input text

[hamorillo]

The behavior of the placeholder can be improved.

Reminder: We can't use experimental composables.

[AdamGrzybkowski]

@hamorillo I wonder if we need this. There's always a default alt text set, so the only way to show the placeholder is to delete the text manually. At that point, the placeholder doesn't provide much value.

[hamorillo]

Yeah, I agree. With the new default altText, the placeholder doesn't add much value. We can reduce the priority to low (just in case, at some point, we have time) or remove the ticket.