Customisable laser properties

This allows laser properties to be customised by OXP, and new laser weapons to be defined. Should work with any equipment starting "EQ_WEAPON_", and the weapon_info dictionary sets its properties (all values have defaults to pulse laser)

It also includes some modifications I've been playing with to the beam/military laser spec which I think give much more interesting fights and sort out some of the balance issues.

Also included: weapons can have default colours set, which are overridden by the ship laser colour. At the moment I've spread the weapon colours out a bit to demonstrate+test this, but we could put the defaults back to red easily enough.

Current incompatibilities:
 - weapon_energy doesn't work for setting NPC forward laser damage any more (the moment we made lasers writable by script that property became somewhat silly, and the obvious new method is to define a suitable laser weapon for the ship). The other use of weapon_energy is for the old missile explosion routine which is already superseded, so this is perhaps as good a time as any to deprecate that property fully.
 - the old "twin plasma cannon" isn't there any more. I doubt anyone is going to miss it. At some future point extending the weapon customisation code to allow projectile and plasma weapons should be possible, but that would require teaching the NPCs how to lead their shots - the code is already in the plasma turret methods, of course - and perhaps about ammunition counts, which is well outside the scope of this code.

Closes #21