Do no rely on CellRenderer mode but on type

The mode value is from the last row rendered not the current one.
So we must just rely on the type of CellRenderer to know if it can be edited.

issue8483
review261821002

tryton/gui/window/view_form/view/list_gtk/editabletree.py

@@ -13,8 +13,10 @@ _ = gettext.gettext
 
 def focusable_cells(column, editable=True):
     for cell in column.get_cells():
-        mode = cell.props.mode
-        if not editable or mode & Gtk.CellRendererMode.EDITABLE:
+        if not editable or isinstance(cell, (
+                    Gtk.CellRendererText,
+                    Gtk.CellRendererCombo,
+                    Gtk.CellRendererToggle)):
             yield cell