Add tag-and-drag setting from libinput into mutter

The problem is that libinput offers the possibility to not enabled
dragging when tap-to-click is enabled but mutter doesn't. For people who
have a sensitive touchpad and who like tap-to-click option, dragging is
launched even when you don't want it : for example, when you select a
folder, most of the time the folder is dragging whereas just selected or
when you want to select some lines of a text file, several lines are
moved as a cut-paste which is not expected and erase datas.

To fix it, you need to have the possibility to desactivate the drag
option when you use tap-to-click in mutter. Because it's already a
specification of libinput, it remains to add it to mutter.
Implementation with X11 is added too.

https://bugzilla.gnome.org/show_bug.cgi?id=775755

src/backends/meta-input-settings-private.h

@@ -54,6 +54,9 @@ struct _MetaInputSettingsClass
   void (* set_tap_enabled)   (MetaInputSettings  *settings,
                               ClutterInputDevice *device,
                               gboolean            enabled);
+  void (* set_tap_and_drag_enabled) (MetaInputSettings  *settings,
+                                     ClutterInputDevice *device,
+                                     gboolean            enabled);
   void (* set_disable_while_typing) (MetaInputSettings  *settings,
                                      ClutterInputDevice *device,
                                      gboolean            enabled);