display: Expose window_grab_modifiers

Since commit c255031b6d we pass scroll-events through to
the compositor if the window_grab_modifiers are pressed;
in order to allow gnome-shell to check for those events,
expose the struct member as a MetaDisplay property.

Also take the opportunity to pick a more generic name, now
that the modifier is no longer used exclusively for mouse
clicks (unless we maintain the notion of scroll events as
button 4 and 5 "clicks").

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1695>

src/core/keybindings-private.h

@@ -141,8 +141,6 @@ gboolean meta_keybindings_process_event     (MetaDisplay        *display,
                                              MetaWindow         *window,
                                              const ClutterEvent *event);
 
-ClutterModifierType meta_display_get_window_grab_modifiers (MetaDisplay *display);
-
 gboolean meta_prefs_add_keybinding          (const char           *name,
                                              GSettings            *settings,
                                              MetaKeyBindingAction  action,