Add ability to set a window's parent using BrowserWindowOptions

[lukebordonaro]

Closes #666

[lukebordonaro]

I initially only edited browserWindows.ts and ran tsc, but it resulted in a lot of changes to js and js.map files. I decided to make the necessary changes to browserWindows.js manually for this PR - let me know if you'd like me to run tsc instead.

[arun04cbe]

Hi @GregorBiswanger @MutatedGamer
The code for making the popups modal has been merged but when I check in the repo, it seems the code is not reflected.
Can you kindly check this to get the modal popups working?
(FYI - This has not been reflected in the latest Electron.NET version)