Enhance bar tabbing, retain _ on hi'ing selection

Correctly allow Tabs to propagate to the bar level only, so they can be handled there
Add a special case for tabbing from the last term input in case the term-append button is hidden (so needs focusing manually)
Do not remove connector punctuation (Pc) when getting terms from selection (https://en.wikipedia.org/wiki/Unicode_character_property)

Fixes #129

searchmarkers / mark-my-search