Use sheet-end event to handle focus when closing macOS file upload dialog

[yuya-oc]

Before submitting, please confirm you've

• read and understood our Contributing Guidelines
• completed Mattermost Contributor Agreement
• executed npm run lint:js for proper code formatting

Please provide the following information:

Summary
Keep focus when closing file upload dialog on macOS.

This requires Electron 1.6.8 which is still beta.

Issue link
#341

Test Cases
On Mac:

1. Upload file using CMD or CTRL + U
2. Choose file with keyboard + hit enter to select
3. Hit enter to send the message

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/240#artifacts

[jasonblais]

Sorry @yuya-oc I forgot about this one. I'll ask someone on our team to help test it.

[lindalumitchell]

@jasonblais @yuya-oc Looks great! I tested on OSX on the test build here: https://240-67276967-gh.circle-artifacts.com/0/tmp/circle-artifacts.dj6jLhv/mattermost-desktop-3.7.0-mac.tar.gz and in all cases focus stayed in the text box during and after attachment upload.

Tested using CMD-U, copy/paste, drag-and-drop, and paper clip icon. Tested center and RHS, single and multiple attachments, with and without text in the post, desktop view and mobile view. Tested clicking to submit file choice and pressing Enter to submit. Tested viewing file preview and downloading attachments. All looks good; no issues found.

[jasonblais]

1.6.8 is no longer in Beta so it might be okay to merge this.

[yuya-oc]

@jasonblais Rebased due to conflicting of package.json. The changes were already tested. So I think we can merge this after simple testing.

https://circleci.com/gh/yuya-oc/desktop/269#artifacts

[jasonblais]

Sounds good, I'll ping Linda next week. Great to have this fixed :)

[yuya-oc]

Tested by Linda. Thanks!