Blender Git Loki

Collision: allow disabling collision without removing the modifier.

The `object.collision.use` flag was treated as a redundant marker
of the existence of the modifier, going as far as adding/removing
it when the value was changed, which is not actually very useful.
Removing the modifier loses its position in the stack, and requires
a dependency graph rebuild. It feels it may be a legacy flag?

What would be useful however is the ability to toggle collisions
dynamically without removing the modifier. This patch adjusts the
code to only add the modifier when the flag is enabled. Disabling
the setting on the other hand keeps the modifier, which now checks
the flag at the start. The redesigned setting is exposed in the UI.

Collisions can't be disabled by simply using the modifier enable
flags because it still needs to be able to delete the stale data.