Added MacOS support for 3.0

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Motivation

I wanted to get awakened_poe_trade working on Mac, and this was the biggest barrier, since no binaries existed for this on Mac.

PS: thanks a lot for the great demo app! I don't think developing this would've been possible without that

Fix

Created Mac interface for overlay_window.h - for the most part, this is structured similarly to the Windows/Linux code, with a few exceptions:

1. Mac doesn't allow windows to be attached/reparented to those of other applications, so the overlay window has to stay within the separate app. To make this not look weird, we hide the window whenever the target window isn't on top
2. Mac doesn't give a global way to listen to windows becoming the foreground window. We instead do 2 things:
   • Attach to the foreground window and detect when it gets deactivated or destroyed. When it does that, attach to the next foreground window
   • When attaching observers, we poll for 5 seconds since the observers seem to take a while to take effect

Side changes

• Documented the general structure of the code a bit in DEVELOPING.md
• Changed the example to work with TextEdit on Mac too and changed the keyboard shortcuts to not conflict with system shortcuts (Cmd + H on Mac is hide, Cmd + Q is quit app)

Testing

Tested by:

1. Run yarn demo:electron
2. Open an empty, untitled TextEdit window. If you need to change the sample window, edit attachByTitle in the electron-demo file to attach to a window with a different title

Tested

• Getting permission
• Blurring/focusing
• Closing the app
• Only opening the window after startup
• Keyboard shortcuts

Resizing, focusing

Fullscreen

https://user-images.githubusercontent.com/2937410/119969235-29ddc780-bf63-11eb-90d0-d80e57dbf5c5.mp4

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@SnosMe - would it be possible to just merge this in? That would save me the tedious work of having to rebase once in a while. You don't have to address the Mac bugs and can just @ me for them and I can take a look.

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Here's a new recording of the latest test - note that it actually works on fullscreen too, but my screen recorder (liceCAP) is broken with it.