Metatile behaviors with no constant are not handled

[GriffinRichards]

If a metatile has a behavior value that doesn't have a corresponding constant then the behavior combo box in the Tileset Editor is just displayed empty. It should instead display the behavior value (and allow direct value entry). This might be considered user error, but Porymap should probably handle it correctly regardless (and this actually happens in current pokefirered, fixing that now).

This same issue probably applies to other combo boxes, and any that allow users to enter their own values should be fixed as well.