- 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.
Arrow signs are specific to submenu items, so they would be more natural
to be handled in menu.c rather than accepting "arrow" in
font_buffer_create().
Also I allowed non-positive numbers for max_width in font_buffer_create(),
in which case the natural font width is used as the buffer width.
