Move Main.currentTime() and Main.createAppLaunchContext() into ShellGlobal

Primarily motivated by wanting these functions accessible from C.

https://bugzilla.gnome.org/show_bug.cgi?id=603522

src/shell-global.h

@@ -82,6 +82,9 @@ ClutterModifierType shell_get_event_state (ClutterEvent *event);
 void shell_popup_menu (GtkMenu *menu, int button, guint32 time,
                        int menu_x, int menu_y);
 
+guint32 shell_global_get_current_time (ShellGlobal *global);
+
+GAppLaunchContext *shell_global_create_app_launch_context (ShellGlobal *global);
 
 G_END_DECLS