Blender Git Loki

UI Code Quality: Start refactoring Outliner tree-element building (using C++)

Continuation of the work started with 249e4df110e0. After all display modes
were ported to this new design, this commit starts the (more complex) work on
the individual tree-element types. More concretely it ports animation
tree-elements (action data-blocks, drivers and NLA data).

The commit above explains motivations. In short, we need a better design that's
easier to reason about and better testable.

Changes done here are pretty straight forward and introduce similar class
hierarchy and building patterns as introduced for the display modes already.
I.e. an abstract base class, `AbstractTreeElement` with derived classes for the
concrete types, and a C-API with a switch to create the needed objects from a
type enum. The latter should be replacable with something nicer later on (RAII
based, and type-safer through meta-programming).
Each tree-element type has its own class, with an own header and source file
(okay some closely related types can share a header and source file, like the
NLA ones).

I added some further temporary bits for the transition to the new design, such
as the `TreeElement.type`. It should entirely replace `TreeElement` eventually,
just as `outliner_add_element()` should be quite small by then and easily
replacable by a `TreeBuilder` helper.