Currently all units that can build can also gather all resources, and vice versa; those units have the Worker class. While this works for the public mod, there is no compelling reason this will always be the case: units that can build but not gather are perfectly conceivable, as are units that can gather but not build.
This patch introduces a Builder visible class, which may make it clearer for (new) players to understand which units can build. This is also the class that should be used by technologies (currently none) and auras.
Furthermore, it introduces an “idle builder” hot key, which ought to make it easier to find units to build structures, and assigns the semicolon for it, due to its proximity to the period (idle worker) and slash (idle warrior), and because that key is not used yet.