This commit adds new theme options:
- osd.window-switcher.style-classic.item.active.border.color
- osd.window-switcher.style-classic.item.active.bg.color
These theme options configures the border/background of selected window
item in the `classic` style window switcher. Their default values are
identical to `thumbnail` style window switcher, which means the default
border color is now `osd.label.text.color` with 50% opacity and the
default background color is now `osd.label.text.color` with 15% opacity.
- 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.
