AllegroDotNet v0.9.0

A very large refactor of the backend code, but the exposed API changes are small.
They are mostly limited to some classes becoming structures, and requiring ref when they are passed.

AllegroDotNet can now handle different versions of Allegro 5, either by using the LibraryVersion enumeration when calling Al.InstallSystem(), or by passing the integer version. The SubC.AllegroDotNet.Win64 NuGet package is updated with Allegro v5.2.9.