Blender Git Loki

UI Code Quality: General refactor of Outliner View Layer display mode creation

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).
* Move important variables to classes rather than passing around all the time
(shorter, more task-focused code, localizes important data names).
* Add helper class for adding object children sub-trees (smaller, more focused
units are easier to reason about, have higher coherence, better testability,
can manage own resources easily with RAII).
* Use C++ iterators over C-macros (arguably more readable, less macros is
generally preferred)
* Add doxygen groups (visually emphasizes the coherence of code sections,
provide place for higher level comments on sections).
* 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).