Machine interface tags

[tatjam]

There are some cases where a machine interface needs to carry a bit more information. For example, activable machines may be meant to be staged (such as the engine ignition), or activated at certain moments of the mission (for example, landing gears). In some cases, it may not be clear which one is appropriate (for example, a fluid pump).

This can be fixed by having machine interface specify a series of tags, which can give a default value but can also be modified by the user.

The purpose of these tags is to simplify greatly the auto-wiring mechanics. See #38

[tatjam]

The interface to control which tag is used could be accessed from the editor via right-click.