Ensure that state costs are drawn in black.

Previously, the state cost would be drawn using the color used to draw the
number of the last tile. When the last tile was empty (represented by a
dash) the color of the state costs values would be grey.
Change Tree View have separate X and Y margins.

The allows the X margin size to be larger (120px), ensuring that the root
node is not obscured by the Stats View when using DFS.