Media: add semantic indication for the buttons group active state

[afercia]

In the media views, there are a few places where "buttons groups" are used. For example: the alignment setting, the video preload setting, the "Insert from URL' media settings:

The active button gets an .active CSS class, which changes the visual and makes the button look active.

However, there's no semantic indication at all of the button state. Assistive technologies and any other software won't have a clue what the active setting value is.

The aria-pressed state would help in this case. Although, according to the ARIA recommendation, it should be used on buttons that "require a full press-and-release cycle to change their value", it's the only ARIA attribute that can be used on buttons to communicate their active state.

[afercia]

48355.diff​ adds the aria-pressed attribute to the media buttons group. This way, buttons in a group are announced as "toggle button" (the announcement may vary depending on the screen reader):

And the active button is announced as "selected":

[afercia]

In 46748:

Accessibility: Media: Add an aria-pressed attribute to active button within a button group.

The aria-pressed attribute communicates semantically the "active" state of buttons that otherwise only look "active".

Fixes #48355.

[afercia]

Reopening for 5.3.1 consideration.

[SergeyBiryukov]

In 46778:

Accessibility: Media: Add an aria-pressed attribute to active button within a button group.

The aria-pressed attribute communicates semantically the "active" state of buttons that otherwise only look "active".

Props afercia.
Merges [46748] to the 5.3 branch.
Fixes #48355.