Add unit baseclasses for triggers/effects and neutral objects

[gajop]

The following unit baseclasses should be added:

• Triggers/effects: invisible, neutral, non-collidable objects that are used as part of spawn/trigger mechanics or ways to generate LUPS effects.
• Neutral objects that units won't shoot at. In addition there should be a way (in customParams) to define whether these objects are 1) selectable/clickable, 2) collidable, 3) invulnerable.

These changes will also require creating basic gadgetry to do the necessary Spring callouts that set these fields.
The "invulnerability" of these objects can be handled via a callin.