Add experimental popup window support

[PalmerAL]

This adds support for opening popup windows from webpages, using this new feature of Electron. Implementing this would fix most of the compatibility issues mentioned in #140

Known issues:

• ctrl+clicking on links to open them in a new tab no longer works. To fix that, we need feat: add more info in setWindowOpenHandler details electron/electron#28518, which requires upgrading to Electron 14 (or maybe 13?)
• Occasionally the browser crashes when closing the popup window.
  • It would be really helpful to find a consistent way to reproduce this - if anyone finds one while testing, please let me know.

[PalmerAL]

CC @blackgwe - this fixes the issue with the hypothes.is login page.

[blackgwe]

@PalmerAL Thank you very much. It works for me. Unfortunately, I don't know any other page with pop-up windows where I can test the browser crash. If I can help, pls let me know!

I tested minbrowser with current electron 14.0.0-nightly.20210413 successfully, but the "ctrl+clicking on links" bug does not disappear.

Another idea: The "ctrl+clicking on links" bug shows that it's possible to support multiple minbrowser windows (of course it's confusing to have tasks, tabs and windows…) But maybe you can have a config value in the min://settings) to enable "windows support" which means, there is a menu item "File Menu" → "Open new window".
-> #1582
-> #1573

[PalmerAL]

Great!

Once we upgrade, I think we need to add a windowOpenHandler (https://www.electronjs.org/docs/api/web-contents#contentssetwindowopenhandlerhandler) and then it should probably work.

Multiple windows would be nice, but there's a lot of features of Min that won't work without substantial work to modify them (I listed some of them here: #169 (comment)). Fixing all of those problems hasn't been a priority, and so I don't think it makes sense to have a hidden setting for something that doesn't really work.

[PalmerAL]

ctrl+clicking is fixed now, and I'm not seeing any other crashes (or any other issues really), so I'm hoping this is done.

I'll probably test this for a little bit, and assuming no issues, merge and release a beta version with this change.