Fixed how trees react to explosion.

Added doingExplosion tag to TETreeLog so the ProcessTree method only
gets called one time.
Leaves also use TETreeLog to save the base data now.
You can only chop a tree down by breaking one of the bottom tree blocks
or 1 above.
Floating leaves and trees will disappear now.
Still need to add leaves path finding to check if they are connected to
a log block.
Logs and leaves will check if the base TE is still there, and if it's
missing they will dissapear when they get updated.