Fix gray line defect

[pronebird]

Describe what this PR changes. Why this is wanted. And, if needed, how it does it.

Git checklist:

• Describe the change in CHANGELOG.md under the [Unreleased] header.
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This PR fixes the misplaced gray line rendered above the app window on macOS. The upgrade to Electron 4.2.3 was necessary due to some PRs that landed in the release and made it possible to workaround the issue.

v2019.4:

this patch:

Please make sure that you can quit the app by clicking the "quit" button in Settings, this is due to some internal workarounds that temporarily make window non-closable to be able to hide the traffic lights in titlebar.

Closes #541

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This change is 

[jvff]

Tested here and the line is gone and the Quit button still works as expected :)

Reviewed 4 of 4 files at r1, 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved