Customizer Widgets and Themes search improvements

[afercia]

Widgets and Themes search would need the same improvements done for the Menu Customizer.

Widgets:

• activating "Add a Widget" with the Space bar doesn't prevent the default action, as a result the search field gets a space character and the placeholder text is gone. See ​https://github.com/voldemortensen/menu-customizer/issues/107

Both:

• the search results (or no results) should be announced to assistive technologies, possible use case for wp.a11y.speak see #32720
• a short description of the "live" search functionality could help, see the Menu items search in core, i.e.: aria-describedby="menu-items-search-desc"

[valendesigns]

There really should be more parity in the Customizer, whether that be functionality or accessibility it should all work relatively the same. I'll take this one on in 4.4.

[afercia]

The first widgets part was fixed in [35304]. Still to do: the search for both Widgets and Themes.

[afercia]

Related: #32882.

[afercia]

For history: the parts mentioned here as still to address (the search for both Widgets and Themes) were actually fixed in [38709] for the Widgets and in [41648] for the Themes, with the introduction of the new "Themes browser".