Implement run_embedded and schedule_modal_fn on Windows

[Osspial]

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior

Implements request 2 in #159 (comment) and the API mentioned in #638 (comment). This brings us halfway to where we need to be to allow users to create DAW plugins on Windows, the other half being to implement embedded windows as discussed in #403 and #159.

This is a draft PR because it depends heavily on the work in #1496.

[kunalarya]

@Osspial -- this is really neat!

I wanted to share some work I've been doing to get child window support for MacOS. The key things I've added is a EventSubscriber object, which looks functionally similar to your EventLoopEmbedded class?

In any case, I've plumbed these changes into a glutin branch and was successfully able to create child windows in my DAW host. I should note that I'm not a GUI dev, and I'm sure I'm doing something horrible unsafe on this branch; however, it does sketch out a rough prototype:
master...kunalarya:child-window-mac-os

I'm totally fine updating my branch to use the same or similar API. What's your feeling on the direction here, re child windows?