Add a proximity upgrade source, and TS stealth generator

This introduces a proximity upgrade source and implements the stealth generator as a demonstration of its capabilities.  This will also be useful for area buffs (like the propaganda tower in C&C generals) or debuffs (see [gapgen-sight](https://github.com/pchote/OpenRA/commits/gapgen-sight) for a prototype of the gap generator sight restriction).

~~The way that our production code is set up means that the proximity bonus won't be granted until the newly produced actor's second tick in the world, which means that units produced in the stealth field will be visible for their first tick.  This will probably require some reworking of the production code to fix.~~