Improve handling of Attachable terrain collision while attached

[garethyr]

Is your feature request related to a problem? Please describe.
Currently you have to enable terrain collisions in ini to have them be toggleable. This should be changed so you can toggle them at any time. This also only works for attachables, where it should be available for anything that collides with terrain.

Describe the solution you'd like

1. Should be able to toggle terrain collisions for anything that collides with terrain. I guess this is either MOSR or MOSprite. For attachables it should be extended to add their atoms to their parents if they're attached. This'll probably require some separate list storing enabled atoms or something, and may be complicated.

2. Should be able to toggle terrain collisions regardless of whether they were initially turned on in INI.