Show labels explaining why a unit can't be built.

The possible labels are NO SUPPLY and NO POWER.
They don't get updated properly without /luaui reload yet.
Update supply/power labels when status changes.
Disable units that go over supply on game start.

This fixes the issue where right after start turrets costing supply
don't get greyed out in integral menu, which is confusing.
Merge pull request #9 from db81/integral_labels

NO SUPPLY and NO POWER labels in integral menu.