Some further cleanup and 1.13 cherry-picks

Are most of these changes small and petty? Yes. However, IMO they do
make the code more readable. Anyway, a summary of some of the more
interesting changes:

 - Expose Abstract*Upgrade classes in the API
 - Fix the spelling of Jonathan in the API docs (*shakes fist*)
 - Fix bug with printout not working in the offhand.
 - Rename any argments/variables accidentally named "m_*", and add an
   inspection to prevent it happening again.
 - Remove most of the Block*.Properties classes - just inline them in
   the parent class.
 - Return super.writeToNBT instead of reassigning at the top.

src/main/java/dan200/computercraft/core/apis/FSAPI.java

@@ -34,9 +34,9 @@ public class FSAPI implements ILuaAPI
     private IAPIEnvironment m_env;
     private FileSystem m_fileSystem;
 
-    public FSAPI( IAPIEnvironment _env )
+    public FSAPI( IAPIEnvironment env )
     {
-        m_env = _env;
+        m_env = env;
         m_fileSystem = null;
     }