Added "Tool Skill" display

I added the display of tool skills. If the unit has an item in their possessions that has the same effect as a skill just by holding it, that skill will now be displayed on the stats and the unit list screen.

"Tool Skill" will be displayed in the skill description window.

There are currently two items with this display, Knight Ring and Nihil Ring.

Once I start doing things like this, it is inevitable that some players will request to display unique skills for items like Balmung (Shield), but there are no plans for that at the moment.