allure plugin for robotframework-seleniumlibrary

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#### I'm submitting a ...
- [ ] bug report
- [x] feature request
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This could be an good example or part of a robot adapter
```python
import allure
from SeleniumLibrary import ScreenshotKeywords
from SeleniumLibrary.base import keyword, LibraryComponent

class AllureSeleniumPlugin(LibraryComponent):

@keyword
def capture_page_screenshot(self, filename='selenium-screenshot-{index}.png'):
path = ScreenshotKeywords(ctx=self.ctx).capture_page_screenshot(filename)
allure.attach.file(path, attachment_type=allure.attachment_type.PNG)
```

```robot
Library SeleniumLibrary plugins=AllureSeleniumPlugin
```

Start with the proposed Python AllureSeleniumPlugin example and its capture_page_screenshot entry point. Check how the Robot Framework Library setting passes plugins=AllureSeleniumPlugin, then determine the integration's required scope. Done should provide a usable SeleniumLibrary plugin that attaches captured page screenshots to Allure reports.

Written by the indexing model from the issue text.