Move WindowPreviewLayout from JS to C

This layout manager is used quite often and the time we spend calling
it's allocate and get_preferred_width/heigth functions increases with
every open window.

We can save a lot of precious time during the layout cycle by moving
this layout manager from JS to C, thus avoiding the overhead of
trampolining between C and JS land.

In a measurement where the average time spent in vfunc_allocate() of the
Workspace actor was measured while opening an overview with 20 windows,
the average time spent went down from 3.1 ms to 2.3 ms.

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

src/shell-window-preview-layout.h

@@ -0,0 +1,33 @@
+#ifndef __SHELL_WINDOW_PREVIEW_LAYOUT_H__
+#define __SHELL_WINDOW_PREVIEW_LAYOUT_H__
+
+G_BEGIN_DECLS
+
+#include <clutter/clutter.h>
+
+#define SHELL_TYPE_WINDOW_PREVIEW_LAYOUT (shell_window_preview_layout_get_type ())
+G_DECLARE_FINAL_TYPE (ShellWindowPreviewLayout, shell_window_preview_layout,
+                      SHELL, WINDOW_PREVIEW_LAYOUT, ClutterLayoutManager)
+
+typedef struct _ShellWindowPreviewLayout ShellWindowPreviewLayout;
+typedef struct _ShellWindowPreviewLayoutPrivate ShellWindowPreviewLayoutPrivate;
+
+struct _ShellWindowPreviewLayout
+{
+  /*< private >*/
+  ClutterLayoutManager parent;
+
+  ShellWindowPreviewLayoutPrivate *priv;
+};
+
+ClutterActor * shell_window_preview_layout_add_window (ShellWindowPreviewLayout  *self,
+                                                       MetaWindow *window);
+
+void  shell_window_preview_layout_remove_window (ShellWindowPreviewLayout  *self,
+                                                 MetaWindow *window);
+
+GList * shell_window_preview_layout_get_windows (ShellWindowPreviewLayout  *self);
+
+G_END_DECLS
+
+#endif /* __SHELL_WINDOW_PREVIEW_LAYOUT_H__ */