Improve accessibility of the Trac ticket Keywords and Focuses

[afercia]

I'd like to propose a few improvements for the Keywords and Focuses user interface within the ticket properties section. Here's how they currently look like:

Color contrast (personally, I struggle to see the focuses buttons text), and many other things can be improved, for example:

• the "manual" link is barely visible
• it can't be operated with a keyboard
• it should be a button
• accessibility of the "X" buttons to remove keywords: they're just empty links
• keyboard interaction with the custom keywords input field can be improved
• focuses buttons color contrast can be improved
• focuses buttons should be buttons
• their "pressed" state should be communicated semantically

Patch incoming. Please consider I don't have a running local environment for testing. I've managed to build a SVN diff and would greatly appreciate some testing. Any feedback is welcome.

[afercia]

Trying to recap what 4495.diff​ does:

"Manual" button:

• changes the link to a button element
• adds an aria-label "Manual keyword" to clarify its purpose
• adds an aria-expanded attribute to make screen readers users understand something in the page opened
• styles a bit the button and improves color contrast

Remove keywords "X" buttons:

• changes the empty links to buttons with meaningful aria-labels
• handles focus to avoid focus losses when removing the keywords

Custom keywords input field:

• adds an aria-label to label the field
• handles keyboard interaction when pressing Enter and Escape: both Enter and Escape populate the keywords and hide the field
• attaches the field change event on initialization: previously it was attached each time the "manual" button was clicked

Focuses:

• makes this section look the same on the new ticket and on the ticket pages: previously they looked different (see attached screenshots)
• wraps the buttons in a fieldset element with a legend "Focuses:"
• makes the buttons real <button> elements
• adds an aria-pressed attribute to communicate the buttons state
• styles the buttons to improve color contrast and provide better hover / focus styles

General:

• cleans up a bit the CSS

Note: I'm assuming there's no need for proper l10n like in core, as Trac is always in English.

/Cc @SergeyBiryukov

[afercia]

New ticket before

[afercia]

New ticket after

[afercia]

Ticket page before

[afercia]

Ticket page after

[SergeyBiryukov]

In 8973:

Trac: Improve accessibility of keywords and focuses on Trac ticket pages.

Props afercia.
Fixes #4495.

[SergeyBiryukov]

In 8974:

Trac: Bump scripts version after [8973]. See #4495.