Use screen methods to activate or not the next/previous buttons of attachment preview

Closes #12756
(grafted from cf043bb57421ab47f68cf8cc41745489cf0187d4)

--HG--
branch : 7.0

sao/src/tab.js

@@ -1283,8 +1283,8 @@
             preview.record_message = function(position, length) {
                 var text = (position || '_') + '/' + length;
                 label.text(text).attr('title', text);
-                but_prev.prop('disabled', !position || position <= 1);
-                but_next.prop('disabled', !position || position >= length);
+                but_prev.prop('disabled', !screen.has_previous());
+                but_next.prop('disabled', !screen.has_next());
             };
             screen.windows.push(preview);
 

tryton/tryton/gui/window/form.py

@@ -219,8 +219,8 @@ class Form(TabContent):
         class Preview():
             def record_message(self, position, length, *args):
                 label.set_text('(%s/%s)' % (position or '_', length))
-                but_prev.set_sensitive(position and position > 1)
-                but_next.set_sensitive(position and position < length)
+                but_prev.set_sensitive(screen.has_prev())
+                but_next.set_sensitive(screen.has_next())
         screen.windows.append(Preview())
 
         vbox.pack_start(screen.widget, expand=True, fill=True, padding=0)