Adds support for additional keywords in weapons' TipImage.

e.g

MySkill = Skill:new{
	TipImage = {
		Sand = Point(1,1),
	}
}

New supported keywords:

(This PR was made very quickly, so this is not a complete list of keywords that the vanilla game don't support. Feel free to mention other missing keywords)

• Shield
• Sand
• Ice
• Frozen
• Snow

To have multiple features with the same key, the keyword can be suffixed with numbers.

TipImage Callbacks

This feature allows the optional definiton of a TipImageCallbacks table for a weapon, which associates specific elements in the weapon's TipImage table to callback functions in TipImageCallbacks.

Example:

TipImageCallbacks = {
	BuildingSmall = function(loc)
		Board:SetTerrain(loc, TERRAIN_BUILDING)
		Board:SetHealth(loc, 2, 2)
	end,
	BuildingDamaged = function(loc)
		Board:SetTerrain(loc, TERRAIN_BUILDING)
		Board:SetHealth(loc, 1, 2)
	end,
}

This example shows a TipImageCallbacks table with two custom callbacks that can be referenced by the weapon's TipImage; either by the same name or the name with a numbered suffix.

Example Weapon:

MySkill = Skill:new{
	Name = "MySkill",
	Description = "MyDescription",
	TipImage = {
		Unit = Point(1,1),
		Enemy = Point(1,2),
		Target = Point(1,2),
		StructureSmall = Point(2,1),
		StructureDamaged = Point(2,2),
		StructureDamaged2 = Point(2,3),
	},
	TipImageCallbacks = {
		BuildingSmall = function(loc)
			Board:SetTerrain(loc, TERRAIN_BUILDING)
			Board:SetHealth(loc, 2, 2)
		end,
		BuildingDamaged = function(loc)
			Board:SetTerrain(loc, TERRAIN_BUILDING)
			Board:SetHealth(loc, 1, 2)
		end,
	},
}

In this example, the weapon "MySkill" has a TipImage table with a StuctureSmall at Point(2,1). When the tipimage is displayed in-game, the TipImageCallbacks.StuctureSmall function will be called for the TipImage.StuctureSmall element, causing a building with 2/2 health to be created at Point(1,3) on the game board.

It also has a pair of StructureDamaged entries. When the tipimage is displayed in-game, the TipImageCallbacks.BuildingDamaged function will be called for both TipImage.StructureDamaged and TipImage.StructureDamaged2, causing a building with 1/2 health to be created at Point(2,2) and Point(2,3) on the game board.