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".

[celloexpressions]

@afercia what is left to do here? From 8 it sounds like these items are fixed now and this ticket can be retroactively closed as fixed in 4.9 (the themes panel redesign).

Noting that customizer widgets are being replaced with blocks in 5.8, so that piece should be addressed with the upcoming version if it wasn't already fixed.