Add accelerator key to clear or toggle the search highlighting

[Lewis Rosenthal]

Currently, clearing search highlighting requires opening the search dialog again and searching on an empty string. This ticket proposes adding an accelerator (Ctrl-Alt-F) to clear or toggle search highlighting.

Ideally, the current search criteria would be preserved and the highlight would be toggled off or restored, however, it would also be acceptable to just be able to turn off highlighting.