Blender Git Loki

Cleanup: Disentangle outliner active status "get" and "set"

Previously the same functions were used to both set and get the active
state for outliner tree elements. This has quite a few problems.
- It's hard to tell when data is changed or simply read
- It prevents using `const`
- The code is full of if statements, making it longer and less readable.

This commit replaces the `tree_element_type_active` and
`tree_element_active` functions with `_get` and `_set` variants. One
has const arguments and returns the active state, the other deals only
with setting the state. While this refactor results in slightly more
lines of code, the result is much better in my opinion.

This commit also removes unused variables from arguments of the affected
functions.

Differential Revision: https://developer.blender.org/D10232