Fixes #5079.

Fully integrates the new 'hint' property supported for touch layouts within KeymanWeb.

Note that a few styling differences were necessary to accomplish this...

• The text-content for the hint is now applied directly to the 'kmw-key-popup-icon' element, rather than specified in CSS for its :before pseudo-element. Kinda necessary, since the hint is no longer statically defined.
• The bold styling for the font has been dropped except when the classic \u2022 dot is used.
• Whatever the source for the hint's text is, its customized font (if defined) is used for styling. In the absence of that, the keyboard's specified font for text is applied to the hint.

Some example screenshots from some test-oriented keyboards I'm developing:

(defaultHint: 'flick')

(each key has a custom hint value)

And, of course, we mustn't break the classic:

(a pre-existing keyboard defining neither hint nor defaultHint)

@keymanapp-test-bot skip