Don't use text value to know the button icon for issue1550

Fix of b9ec0fab5125
The content of the text widget doesn't reflect always the button image as it
can be set to empty text without changing the image.

tryton/gui/window/view_form/view/form_gtk/many2one.py

@@ -285,15 +285,15 @@ class Many2One(WidgetInterface):
             self.wid_text.set_text('')
             self.changed = True
             return False
-        prev_res = self.wid_text.get_text()
+        current_stock = img.get_stock()[0]
         res = field.get_client(record)
         self.wid_text.set_text((res and str(res)) or '')
         img = gtk.Image()
-        if res and not prev_res:
+        if res and current_stock != 'tryton-open':
             img.set_from_stock('tryton-open', gtk.ICON_SIZE_SMALL_TOOLBAR)
             self.but_open.set_image(img)
             self.tooltips.set_tip(self.but_open, _('Open a record'))
-        elif not res and prev_res:
+        elif not res and current_stock != 'tryton-find':
             img.set_from_stock('tryton-find', gtk.ICON_SIZE_SMALL_TOOLBAR)
             self.but_open.set_image(img)
             self.tooltips.set_tip(self.but_open, _('Search a record'))