Allow bays to carry multiple categories of ship about endless-sky

[Amazinite]

Third approach: create a new root node bay. We'd keep category "bay type" to define which ships can be carried. We'd then have these named bay nodes that specify which ships from the "bay type" category they include. When defining a bay hardpoint on a ship, the type of bay that it is would use the named bay types instead of the category of ship that the bay holds. To make things easy, we could just have "Fighter" and "Drone" bay types where "Fighter" bay types can carry both fighters and drones so that we don't need to change any ship definitions to account for this new behavior.

# The categories of ship that can be carried in bays.
category "bay type" 
 	"Drone" 
 	"Fighter"

# Fighter bays carry both fighters and drones.
bay "Fighter"
	"Fighter"
	"Drone"

# Drone bays carry only drones.
bay "Drone"
	"Drone"

# This "Fighter" bay refers to the "Fighter" bay node instead of the "Fighter" category.
ship ...
	bay "Fighter" x y

For the sake of compatibility with plugins that may have added new "bay type" category entries, we'd need to also fall back onto looking for a category instead of a bay node if no bay node is found.

# Imagine a plugin added a new bomber bay type.
category "bay type" 
 	"Drone" 
 	"Fighter"
 	"Bomber"

# Since vanilla doesn't have a "Bomber" bay node, this would fail if we didn't 
# fall back onto looking at the "bay type" category if no bay node with this name
# exists.
ship ...
 	bay "Bomber" x y

This maintains the customization options of the first approach while maintaining backwards compatibility with save files and plugins like the alternative approach.

[OcelotWalrus]

Definitely a nice addition!

[xX-Dillinger-Xx]

Maybe, change the name of the bays to something like Landing Bays or Docking Bays. I think it makes more sense if the bays are to become universal. Will it make it easier to use with the UI?

EDIT: forget this comment, the 3rd approach sounds great.

[tibetiroka]

Third approach: create a new root node bay. [...]

I really like this idea. We should make sure that these bay nodes can be expanded, so plugins can add new ship types to existing bays without having to redefine the entire node (which would not work if multiple plugins wanted to add new entries to the same bay).