Optimise term count checks, greatly optimise PAINT

[ator-dev]

• Always use experimental PAINT APIs
• Optimise getting term occurrence counts for PAINT
  • Only check for first term occurrence in PAINT when updating term status
  • Only update term tooltips on mouseover
• Optimise mutation observation and updating for PAINT (try to only perform updates once even when multiple mutations happened to the same elements)
• Correctly calculate scroll markers for PAINT even if some elements were not "highlightable" (i.e. anchors and their descendants in Chromium for Paint Worklets)

Fixes #124