Fix desktop notifications not working when the window is minimized from inactive state

[yuya-oc]

Blur webview when the main window loses its focus.

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Summary
Fix desktop notifications not working when the window is minimized from inactive state.

Explicitly blur webview when the window loses focus. Tested on Windows 10 and server v3.8.

Issue link
#522

Test Cases

1. Select any channel for this test (We call it "channel A" for simplicity)
2. Make the app window inactive (ex. by clicking on the other window)
3. Click on the "minimize" button on the title bar of Mattermost window.
4. Send a message to the channel A from the other account.
5. Desktop notifications should appear.

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/271#artifacts

[jasonblais]

Tested on windows 10, pre-release, works as expected. Thanks @yuya-oc 🎉

[yuya-oc]

Thanks @jasonblais !