Implement support for variable controlled buildheight

[Pablete1234]

Allows for the use of a variable to read/write the world's max build height:

Example:

<variables>
  <maxbuildheight id="whatever"/>
</variable>
<actions>
    <action id="increase-buildheight" scope="match" expose="true">
        <set var="whatever" value="min(55,whatever+1"/>
    </action>
</actions>

Calling the action increase-buildheight will add 1 to the current height, to a max of 55