[MacOS] Remove window focus listener on unload

[colin-grant-work]

What it does

On master in Electron on MacOS, we attach a listener for window focus and never release it. As a consequence, if the user refreshes the Electron window, they will see logs indicating that a listener was called an a renderer that has been released. This PR removes that message by ensuring that we remove the listener when the window unloads.

How to test

0. On MacOS
1. Start the Electron application
2. Confirm that the OSX top bar menu appears as expected and the menu items behave as they should.
3. Switch windows (on the same monitor, if you have more than one)
4. Observe that the OSX top bar menu is swapped out as expected and continues to work as expected.
5. Refresh the window one or more times. (Cmd+R)
6. Repeat steps 2-4
7. Observe additionally that your backend logs do not show any messages about callbacks in released renderers.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirm that the changes work well on OSX for me 👍

The app-menu is correctly populated and appears, and also works well when switching applications.

I also confirm that the log from master does not appear:

Attempting to call a function in a renderer window that has been closed or released.
Function provided here: vendors-node_modules_electron_remote_renderer_index_js.js:242:30
Attempting to call a function in a renderer window that has been closed or released.
Function provided here: vendors-node_modules_electron_remote_renderer_index_js.js:242:30
Attempting to call a function in a renderer window that has been closed or released.
Function provided here: vendors-node_modules_electron_remote_renderer_index_js.js:242:30
Attempting to call a function in a renderer window that has been closed or released.
Function provided here: vendors-node_modules_electron_remote_renderer_index_js.js:242:30