Make down the default expand direction for RNA Enum dropdown menu.
RNA Enum is the only list type widget that expands up by default, that deceives user expectations and makes it hard to work with when two different types of dropdowns located next to each other.
What this patch does:
- Removes special treatment of expand direction for RNA Enum.
- Adds layout flip for RNA Enum.
RNA Enum menu assembled in reverse, which is why I have to flip its layout because the default direction is now opposite.