Similar to the existing Efl.Ui.Win "background" part, we want to have custom backgrounds in various widgets. Those would likely be one of:
- Solid color (rectangle)
- Rounded rectangle (evas vg)
- Image (evas or elm image)
The "background" part shall be accessed through the API efl_part(obj, "background") (or "bg", TBD), which means we can provide a set of convenience functions to support direct APIs like efl_file_set for image or efl_gfx_color_set for solid backgrounds.
See also D3625 (deprecated).