windowPreview: Subclass a C actor

Move the get_preferred_width/height() and allocate() vfunc
implementations of WindowPreview to C, subclassing the C GObject from
JS.

This gets us another significant performance gain, allocating a
workspace with 20 windows now only takes 1.2 ms.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1743>

src/shell-window-preview.h

@@ -0,0 +1,14 @@
+#ifndef __SHELL_WINDOW_PREVIEW_H__
+#define __SHELL_WINDOW_PREVIEW_H__
+
+#include <st/st.h>
+
+G_BEGIN_DECLS
+
+#define SHELL_TYPE_WINDOW_PREVIEW (shell_window_preview_get_type ())
+G_DECLARE_FINAL_TYPE (ShellWindowPreview, shell_window_preview,
+                      SHELL, WINDOW_PREVIEW, StWidget)
+
+G_END_DECLS
+
+#endif /* __SHELL_WINDOW_PREVIEW_H__ */