Dynamic Gap Generator / Mobile Gap Generator

Recently we have been frustrated with how gap generators can be abused; one can simply build a gap generator, power it down immediately, and the effect stays (as long as you don't have GPS, of course). This seems like a power management exploit against the way gap generators create and contribute to the shroud.

In response, this pull request changes gap generator shroud creation slightly to make it dynamic; when powered down or destroyed, the shroud that it created disappears as well. This shroud is in addition to the "explored" shroud, so if a player has not explored an area, they will not gain exploration simply because a gap generator has been destroyed.

This, of course, paves the way to enable Mobile Gap Generators, which @ScottNZ has graciously been working on recently. The second commit in this pull request enables the mobile gap generators and gives them a shroud creation range of 4.