Split forward/aft shield strengths and recharge for player

Make them writable with JS
Start on scriptable equipment functions in JS with the shield boosters
(the NPC versions stay hard-coded for now, and might as well continue
to do so until/unless NPC shields get implemented)
equipmentAdded / equipmentRemoved generic events for ships

Adjust to make use of these events.
Clean up script startup slightly
Move energy units into scripted configuration
Scriptable control over heat shielding equipment items
Scripts for large cargo bay. ship.cargoSpaceCapacity now writable
Allow galdrive, escape pod, dock comp to be provided.
S.injectorBurnRate and S.injectorSpeedFactor now writable

Allows different ships to be given different injector performance.
S.scanDescription property for customising STE display
Allow conditions scripts to update equipment sale price

this.updateEquipmentPrice(eqKey, price) returns price
Called before multiplier for station.
Correct shield HUD gauges if max shield strength is zero.
Merge pull request #121 from OoliteProject/more-equipment-customisation

More equipment customisation