Integrated targeting system.

Price, description, whether the equipment exists in the core plist at all ... for now somewhat provisional.
Merge branch 'master' of github.com:OoliteProject/oolite into extra-hud-features

Need the high-precision coordinates to continue working on this branch
Fix a few bugs with integrated targeting system

Cost and techlevel (and indeed availability) provisional for now. Issue #31
Move target memory to weakrefs

Fixes previously unreported (but in retrospect obvious) bug where targets you'd never seen before could end up in your target memory.