This allows us to share common codes for dealing with osd items.
For example:
- Get the clicked osd item to focus its associated window
- Scroll the items when their total height is taller than output height
ssd_part_type contains several node types that are not actually part of
server-side decorations (ROOT, MENU, OSD, etc.)
Rename it accordingly and move it to a common location, along with some
related conversion/comparison functions.
- Rename `scaled_scene_buffer` to `scaled_buffer`. This makes it clear
that `scaled_{font,img,icon}_buffers` are implementations of it.
- Move the files from `src/common` to `src/scaled-buffer` as
`scaled_icon_buffer` heavily depends on `server` and `view` etc.
