Blender Git Loki

Cleanup: Better way to pass activate callbacks to Tree-View items

The `ui::BasicTreeViewItem` took a function-like object to execute on
item activation via the constructor. This was mainly intended to be used
with lambdas. However, it's confusing to just have this lambda there,
with no indication of what it's for (activation).
Instead, assign the function-like object via an explicit `on_activate()`
function.