Preview toolwindow improvements (JetBrains#854)

* Activate preview toolwindow on receiving preview configuration

* Don't show preview toolwindow until a preview is requested

* Add icon to preview toolwindow

Update Compose

Update web tutorials for 0.5.0-build228 (JetBrains#833)

Co-authored-by: Oleksandr Karpovich <[email protected]>

Dispose web drivers after selenium tests

Update CHANGELOG.md after releasing 0.5.0-build226

Install gecko driver to the compose-web light Docker image

Add minimal test for testing media query in stylesheet

Update templates