unmount event

[willmcgugan]

This started out as adding an UnMount event, which lead me down a rabbit-hole of how we run and shutdown Textual apps.

It also gave me the opportunity to re-think testing. I've added a run_test context manager which I think is the answer to testing we have been looking for.

I've deleted the older integration tests because I think that a combination of run_test and snapshot testing will be easier to maintain.

@darrenburns I've afraid I've deleted a few lines of snapshot tests where you inspect the app object afterwords, which no longer works with the run refactor. I think the new testing stuff is ultimately more useful, so I hope you won't be too disappointed.

Fixed

• Fixed issue where scrollbars weren't being unmounted

Changed

• DOMQuery now raises InvalidQueryFormat in response to invalid query strings, rather than cryptic CSS error
• Dropped quit_after, screenshot, and screenshot_title from App.run, which can all be done via auto_pilot
• Widgets are now closed in reversed DOM order

Added

• Added Unmount event
• Added App.run_async method
• Added App.run_test context manager
• Added auto_pilot to App.run and App.run_async
• Added Widget._get_virtual_dom to get scrollbars