Hello how add Drawerhost OnOpen/OnClose event like dialoghost DialogOpenedEventHandler/DialogClosingEventHandler

Summary

Many API's are currently lacking XML docs or have invalid markup. As we begin to explore adding API documentation to the Prism Docs, we need to update the existing codebase with more complete XML docs so that these docs can be generated for the docs site.

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Improvement description

Extend the LegendExamples with one that first shows but then hides the legend.
Why? It took me a while to find the IsLegendVisible property.

💻 Environment

Toggl Version: feature/lixux-qml HEAD

🐞 Actual behavior

💯 Expected behavior

![image](https://user-images.githubusercontent.com/3977440/78668824-9b7c3180-78db-11ea-9f0f-e97e7d1c92

Describe the bug
When a timed theme switch is delayed due to high GPU usage, the old monitoring module logged this incident.
The new module has been rewritten more efficiently and does not yet implement this behavior.

Expected behavior
The new GPU monitoring module logs the GPU usage once after the threshold has been surpassed with a message similar to the old module, e.g