This commit adds `<windowSwitcher thumbnailLabelFormat="%T">` to configure the label text in each item in the thumbnail-style window switcher. Its format follows `<fields><field content="custom" format="">`.
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
Before this commit, output transformations were not taken into account for
parcent-based osd widths like:
osd.window-switcher.style-classic.width: 80%
osd.window-switcher.style-thumbnail.width.max: 80%
The if-statement doesn't make sense, because `view_get_string_prop()`
never returns NULL. And if it did, it would cause segfault in
`osd_thumbnail_update()`.
- 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.
