- 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.
This patch also changes the semantics of scaled_icon_buffer: rather than
calling scaled_icon_buffer_set_app_id() every time an app_id is set, we
can now call scaled_icon_buffer_set_view() just once so that multiple
scaled_icon_buffers bound to a window are automatically updated when an
app_id is set or new icon is set via xdg-toplevel-icon-v1.
