add ctrl+backspace to delete word, performance optimizations

[molsonkiko]

1. Ctrl+Backspace in the base form will now delete the word (sequence of non-whitespace, non-punctuation chars) ending at the cursor location (partially address Can't use CTRL+Backspace in search bar #55 )
2. Added optimizations to reduce the frequency of updating of the file list.
3. Added the ability to disable highlighting of the file list if the number of results is large (for better performance, partially address NavigateTo causes NPP to freeze when a huge number of files would be shown #56)
4. Made directory search more efficient by "remembering" whether search terms matched the top directory and thus avoiding the cost of performing unnecessary regex matches.