Add context to import window

Users could expect that importing from a specific window will inherit of his
behavior like the default value of fields. So the context of this window should
be passed to the import_data call.

review2971002
Don't set missing default value to False

False is not always a good valid value for the field, so it is better to let the
field dealing with default value.

issue3643
review3281002
Fix selection of column in next/previous

When _sign is negative, it doesn't select the first column but the second. So
despite of selection the wrong column, it could crash if there is only one
column in the view.

review3241004
Fix dynamic size for translate widget

Dynamic size was added in changeset 8ee7e142c8fd but the translate widget was
not updated.

review3271004