Media template better descriptive strings

[afercia]

Splitting this out from #26550.

Some of the strings used for UI controls in /wp-includes/media-template.php should be clearly understandable even when read out of context. For example, when adding video or audio alternative sources or when adding a video poster image a UI control "Remove" appears. When read out of context it's not clear what this "Remove" refers to.

Please notice new strings were proposed in the first patch on 26550 but then agreed to handle them in a separate ticket for convenience and readability of the patch.

[afercia]

In the proposed patch:

• Media modal: use better descriptive text for the "Remove" source, "Remove" poster image, "Remove" track, buttons.

See screenshot before/after:

[SergeyBiryukov]

In 32972:

Media modal: Use better descriptive text for "Remove" buttons.

props afercia.
fixes #32792.

[ocean90]

Do we still need the context "media" for "Remove video track"? I don't think so.

[afercia]

Remove context for video track

[afercia]

@ocean90: there you go :)

[pavelevap]

Nobody cares about closed tickets :-)

Anybody can commit second patch?

And one more accessibility question: "Remove ... source" is displayed also when there is only one source available? It should not be possible to remove primary (only) source, because there is broken shortcode in the end...

[afercia]

@pavelevap thanks :) we shouldn't re-open a ticket that was closed on a fixed milestone. Do you mind creating a new ticket referencing this one?