add option for noncontiguous search

[periodically-makes-puns]

Resolves #34.

Checking the "Allow non-contiguous matching" box changes the search behavior for all three search boxes. The conditions for a match are as follows:

• every "word" in the search term is a substring of a distinct "word" in the item name,
• the order of words in the search term is identical to the order of the corresponding words in the item name

Issues:

• Flipping the option leads to a brief flickering as a different item name is shown, then immediately replaced.
• Completely doesn't work for Japanese, since no whitespace is used to separate words. Checking the option when the language is set to JP is a no-op.