Fix negative move

I fixed fn.move_widget_down. please merge commit.
There were problem in that case, three 1x1 blocks are at left side and one 1x3 block is at right side. When one block at right side moved to left (1row, 1col), three 1x1 blocks position  become invalid.
That reason is maybe negative y position at fn.move_widget_down. So, i modified to move widget at only y units was positive and returned success value.

See: test/example.html