Blender Git Loki

Cleanup: General cleanup of Outliner Blender File display mode building

See https://developer.blender.org/D9499.

* Turn functions into member functions (makes API for a type more obvious &
local, allows implicitly sharing data through member variables, enables order
independend definition of functions, allows more natural language for
function names because of the obvious context).
* Prefer references over pointers for passing by reference (makes clear that
NULL is not a valid value and that the current scope is not the owner).
* Reduce indentation levels, use `continue` in loops to ensure preconditions
are met.
* Add asserts for sanity checks.