Implement basic crafting skills into recipes/crafting menu

[snipercup]

Requires #191 We have skills defined but they aren't used yet. Here is my implementation idea:

Edit the ItemCraftEditor and add:

• The skill requirement, which is made up of a skill name and a number
• The skill progression which is made up of a skill name and an amount of XP
• The controls should allow the user to drag a skill from the skill list in the contenteditor and drop it on the skill name control

Edit Gamedata to have the skill reference the recipe

• Change the on_item_changed function that will add itself as a reference to the skill that has been configured for the recipe in this item's data
• Modify the on_item_deleted function so that will remove it's reference from the skill
• Add an on_skill_deleted function that will remove itself from an item if the skill is deleted

Edit the crafting menu:

• Hide the recipe if the skill requirement is not met (assuming every recipe is autolearn for now)
• Show the skill name and the amount of xp you will get by crafting the item
• When the item is crafted, add the amount of xp to the player's skill