Display checkbox in multiselection in the filter window

This applies the changes from 97e82c27dc17 to the filter window

sao/src/sao.less

@@ -1261,6 +1261,13 @@ input.column-boolean {
     select[multiple] {
         min-height: 150px;
         max-height: 400px;
+
+        option:before {
+            content: "☐ ";
+        }
+        option:checked:before {
+            content: "☑ ";
+        }
     }
 }
 

tryton/tryton/gui/window/view_form/view/screen_container.py

@@ -159,6 +159,9 @@ class Selection(Gtk.ScrolledWindow):
         self.treeview.set_model(model)
 
         column = Gtk.TreeViewColumn()
+        select_cell = Gtk.CellRendererToggle()
+        select_cell.set_sensitive(False)
+        column.pack_start(select_cell, expand=False)
         cell = Gtk.CellRendererText()
         column.pack_start(cell, expand=True)
         column.add_attribute(cell, 'text', 0)