New MCode and other features for Gdip_ImageSearch

PixelAverage and the search directions for the needle have been removed.
The 4 basic search directions work now for the haystack instead, and the
2 ImageSearch MCodes were unified into a single one. For the next
update, I intend to implement 4 more search directions and fix the Trans
problem, as it's currently working with characters instead of numeric
color values.