scaled-scene-buffer: restructure source files

- 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.

src/common/meson.build

@@ -14,10 +14,6 @@ labwc_sources += files(
   'nodename.c',
   'parse-bool.c',
   'parse-double.c',
-  'scaled-font-buffer.c',
-  'scaled-icon-buffer.c',
-  'scaled-img-buffer.c',
-  'scaled-scene-buffer.c',
   'scene-helpers.c',
   'set.c',
   'surface-helpers.c',