osd: support window switcher with thumbnails

osd: support window switcher with thumbnails

The new-style window switcher can be enabled with
<windowSwitcher style="thumbnail">.

New theme entries:

osd.window-switcher.style-thumbnail.width.max: 80%
osd.window-switcher.style-thumbnail.padding: 4
osd.window-switcher.style-thumbnail.item.width: 300
osd.window-switcher.style-thumbnail.item.height: 250
osd.window-switcher.style-thumbnail.item.padding: 10
osd.window-switcher.style-thumbnail.item.active.border.width: 2
osd.window-switcher.style-thumbnail.item.active.border.color: #589bda
osd.window-switcher.style-thumbnail.item.active.bg.color: #c7e2fc
osd.window-switcher.style-thumbnail.item.icon.size: 60