[AppSwitcher] add a workaround for a gjs problem with GdkModifierType

gdk_display_get_pointer() sometimes returns values for the mask that
aren't part of the GdkModifierType enumeration, which gjs doesn't like
(bug 597292). Work around that by adding a C wrapper that strips out
the extra flags.

https://bugzilla.gnome.org/show_bug.cgi?id=597559

src/shell-global.h

@@ -77,6 +77,8 @@ GSList       *shell_global_get_monitors        (ShellGlobal  *global);
 GdkRectangle *shell_global_get_primary_monitor (ShellGlobal  *global);
 GdkRectangle *shell_global_get_focus_monitor   (ShellGlobal  *global);
 
+GdkModifierType shell_global_get_modifier_keys (ShellGlobal *global);
+
 G_END_DECLS
 
 #endif /* __SHELL_GLOBAL_H__ */