* Switch to .NET Framework 4.7.2 & C# 8.0, update libraries
* Add TweetLib.Core project targeting .NET Standard 2.0
* Enable reference nullability checks for TweetLib.Core
* Move a bunch of utility classes into TweetLib.Core & refactor
* Partially move TweetDuck plugin & update system to TweetLib.Core
* Move some constants and CultureInfo setup to TweetLib.Core
* Move some configuration classes to TweetLib.Core
* Minor refactoring and warning suppression
* Add App to TweetLib.Core
* Add IAppErrorHandler w/ implementation
* Continue moving config, plugin, and update classes to TweetLib.Core
* Fix a few nullability checks
* Update installers to check for .NET Framework 4.7.2
