Allow over-ride of launcher check for linked-fire launchers

[Song]

Currently it is impossible to install a linked-fire launcher without extreme hackery/script-magic to get around the game's installation system being designed to always block multiple launcher installation.

It'd be cool if in future there was a more flexible system where we can state 'this launcher can be stacked with other launchers' (eg. Missile pods with charges rather than external ammo), or "this launcher will stack with itself". I'm not sure how it'd work without potentially throwing massive bugs when you give the wrong inputs, but it'd at least let linked-fire launchers work.

[Song]

Another alternative way to handle this would be to add an extra linked-fire criteria which lets a non-launcher be set to link with the launcher.

[Atarlost]

There shouldn't be any need for any additional attributes.

Nothing should need to change at the XML or script level because the information needed to determine which launchers should be mutually exclusive is already present.

A ship should be allowed at most one primary launcher and as many secondary launchers as it has slots (or as many as the internal representation of a ship can store if they're slotless). Anything with any linkedfire attribute is secondary and anything in a slot with the secondary attribute is secondary.

A launcher installing into a secondary slot or that has a linkedfire attribute would simply not check for existing launchers and a non-linkedfire launcher installing into a non-secondary slot should only check for launchers in non-secondary slots that do not have a linkedfire attribute.