Make the mousewheel a bindable 'key'

The following 'keys' are now available for binding:

• "wheelup"
• "wheeldown"
• "wheelleft"
• "wheelright"

In addition, there are now new default bindings using the scrollwheel:

• "wheelup" in edit mode - previous line
• "wheeldown" in edit mode - next line
• "wheelup" in flight mode - zoom in
• "wheeldown" in flight mode - zoom out

The new zooming binds in flight mode replace the hardcoded zoom from before.

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If you need anything else to accompany this patch, let me know.