#253 a more generic unlocking of buildings

[artoo]

Please make the unlocking of buildings(or units) more generic so mods could unlock a building(or unit) with a founding father.
Atm this seems only possible with the customs house.

[Lone_Wolf]

Taken from freecol/data/rules/classic

definition of custom house

    <building-type id="model.building.customHouse"
                   workplaces="0"
                   upkeep="15">
      <required-goods id="model.goods.hammers"
                      value="160"/>
      <required-goods id="model.goods.tools"
                      value="50"/>
      <required-ability id="model.ability.buildCustomHouse"/>
      <ability id="model.ability.export"
               value="true"/>
      <ability id="model.ability.coastalOnly"
               value="false"/>
    </building-type>

definition of FF stuyvesant

   </founding-father>
    <founding-father id="model.foundingFather.peterStuyvesant" type="trade"
                     weight1="2" weight2="4" weight3="8">
      <ability id="model.ability.buildCustomHouse"
               value="true"/>
    </founding-father>

Robert La Salle is another FF that is related to building : it gives all colonies of size 3+ a stockade .

It should not be hard to use the same method(s) in a mod .