Enhanced character walking - Part 2

Choosing the point where to walk if the user clicks an unwalkable point
now is really faster.
The destination point gets chosen amongst the point of each edge that's
nearest to the source point.
This implied to mantain not only the w-zone quadtree but the original
polygon too.
PLEASE NOTE: right now, clicking inside a polygon hole is note handled
and the character will stay still.
Minor changes: edited the getQuadtreeContainingPoint method, now it can
return even a leaf that's not active, if the "seekOnkyActive" argument
is set to false.