"Launchable" property for item prototypes

[AndreusAxolotl]

With the upcoming ability in 2.1 to increase the maximum weight a rocket can carry (like the 3 ton rocket given as an example in FFF-441), the ability to lock items from being launched into space by giving the item a weight above 1 ton becomes obsolete.

A new boolean "launchable" property could fix the issue. An item with launchable set to false would prompt the rocket silo to display a "Item can't be launched to space" message or something similar.

For example, atomic bombs could have their launchable property set to false instead of having their weight be above one ton.

I believe this addition, while redundant in cases where no rocket silos that can take more than one ton of items exist, would make surface-locking items much more intuitive.