layout: Don't use the input mode to block events to windows

Instead, use the standard Clutter scene graph and our Chrome system.

This also removes our last use of the input mode, so remove that as
well.

https://bugzilla.gnome.org/show_bug.cgi?id=700735

src/shell-global.h

@@ -47,13 +47,6 @@ void     shell_global_end_modal              (ShellGlobal         *global,
 void     shell_global_freeze_keyboard        (ShellGlobal         *global,
                                               guint32              timestamp);
 
-typedef enum {
-  SHELL_STAGE_INPUT_MODE_NORMAL,
-  SHELL_STAGE_INPUT_MODE_FULLSCREEN
-} ShellStageInputMode;
-
-void     shell_global_set_stage_input_mode   (ShellGlobal         *global,
-                                              ShellStageInputMode  mode);
 void     shell_global_set_stage_input_region (ShellGlobal         *global,
                                               GSList              *rectangles);