monsteui LabelSelect/Select with required

```python
from monsterui import all as mu
...
mu.LabelSelect(required=True)
```
or
```python
from monsterui import all as mu
...
mu.LabelSelect(select_kwargs=dict(required=True))
```
Attribiute `required` doesn't work.
When value is not set, form is still sending and marked as validated.