Looting implementation

[KatrinaHoffert]

As described in this forum post. Looting gives and takes resources on death of a unit. This particular implementation allows units to specify what happens when they die: how many resources do they give and how many do they lose. These numbers can be specified as either absolute values or as percentages of their existing resources.

Example:

<parameters>
    <!-- ... -->
    <resources-death>
        <resource name="wood" amount-value="100" loss-value="100" allow-negative="true" />
        <resource name="gold" amount-percentage="10" loss-percentage="10" />
        <resource name="food" amount-value="1" />
    </resources-death>
    <!-- ... -->
</parameters>

As the example shows, we specify a list of resources in a manner akin to the resource cost. The attributes are:

• amount-value: An absolute amount to give to the killing unit.
• amount-percentage: Percentage of the killed unit to give to the faction that killed it.
• loss-value: An absolute amount of resources that are removed from the faction of the killed unit.
• loss-percentage: Percentage of the resource that are removed from the faction of the killed unit.
• allow-negative: If true, the faction can go into the negatives if they lose more resources than they have. If false, the resource count won't go below 0.

All combinations of attributes are valid. All default to 0 (except allow-negative, which defaults to false). Values can be negative to inverse their intent (eg, a negative value for loss-value means that the faction of the killed unit gets some resources back).

Possibilities:

• Implement a werewolf unit that obtains its special "food" by killing units
• Recouperate small amounts of funds by warfare
• When destroyed, buildings return some of their resources

Notes:

• I kind of screwed up this pull request by making it dependent on the starting HP/EP changes. So that pull request must be merged first.