fix Some dropdowns in the flat skin are larger than expected

It was the case for the following widgets:

- Autocomplete,
- Select,
- Selectbox,
- Multiselect
You should put this in `DropDownTextInputCfg`, so that `rightOffsetMargin` appears only there (and perhaps also in `SelectCfg` which does not extend `DropDownTextInputCfg`).
The `MultiAutoCompleteCfg` would then also inherit from this parameter.