[6.x] Improve tooltips

[jaygeorge]

Description of the Problem

• Once upon a time you could copy a field handle by clicking the label
• This behaviour was deprecated some time ago because clicking a label should focus its related input field instead—for accessibility (understandable, but also a shame that we lost this utility!)
• There's some annoying hover-tunnel behaviour where you can't reach any links in a tooltip—like we have for the table headings cp/sites

What this PR Does

• Makes handle tooltips cilckable – which copies the handle to the clipboard
• Fixes some annoying tooltip behaviour – where the tooltip disappears when you move the mouse upwards into the tooltip.
  • Previously if you didn't move the mouse fast enough the tooltip would disappear.
  • The hover gap is now handled by tracking the pointer against the label, the tooltip, and the space between them rather than a hide delay
  • This means you can also reach links in tooltips now

How to Reproduce

1. Try hovering over a field name in a publish form, then clicking the tooltip handle
2. Also try cp/sites and hovering over the little "info" icon