Add ability to selectively whitelist apps

[farmerbb]

This adds an option in the overflow menu for the user to select individual apps to whitelist from the VPN, so that more apps can work around issue #31.

(One caveat: this will require the user to reset settings to default, otherwise, the VPN will not start successfully)

[julian-klode]

Sounds nice. We still have to make sure that the VPN can start successfully without a reset to defaults, though - there should be some migration (that's why I included version numbers, but this can also be done heuristically by just looking if the field is defined or not, and if not, loading it from the default).

I have not looked at the code in detail yet, but I hope to get around to it this week. My schedule so far looks quite OK.

[farmerbb]

Alright, the app whitelist no longer messes with settings.json and should no longer require a settings reset. Let me know if you have any other feedback. I think this will help a lot of users, especially those on Nougat.