Highlight selection plugin: Fix highlighting when same word reselected

[jeremypw]

#1520

• Simplify workings following recent changes to master
• selection-changed signal replaced with selection-event signal (SearchBar no longer listens to original signal)
• Separate deselected signal no longer required
• Anonymous closure used to handle throttled selection-event signal

TODO

• [x] Fix behaviour when search is on going

[jeremypw]

Works OK without ongoing search but regression in draw-spaces when there is a search term in the search bar. Further work required.

[jeremypw]

Blocking as conflicts with #1622 which needs to be merged first.