Implement basic ranged skills into ranged combat

[snipercup]

Requires #191

We have ranged skills but aren't using them yet. This idea describes a basic implementation idea

Edit the ItemRangedEditor and add:

• The skill progression which is made up of a skill name and an amount of XP when using the weapon
• 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

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

Edit the EquippedItem:

• Modify the accuracy calculations to use the skill of the gun that is equipped. This means adjusting the direction in perform_ranged_attack based on the skill of the player. We can say that the player needs level 30 to fire 100% accurate and starts with a minimum accuracy of 25%. This can be represented by a cone of accuracy that gets smaller when it reaches 100%.
• Modify the perform_ranged_attack function to add the xp configured in the weapon to the skill so the player levels up.