add GmailChecker textbox widget

how-to:
add to your config
```python
settings_gmail_checker = {
            'username': "username",
            'password': "password",
            'email_path': "INBOX",
            'fmt': "gmail[%s]",
            'status_only_unseen': True,
        }
screens = [
    Screen(
        top = bar.Bar([
            widget.GmailChecker(settings=settings_gmail_checker),
        ],
]
```