Get it to compile.

* Fix mismatched types between SGB and Slayer sources
* Add NewStrings and Cursors from SGB
Use proper type of casts.

C-style casts won't return null when used on the wrong types, which made
Slayer crash.

Also modify the casted class so it has RTTI info, allowing the use of
dynamic_cast on it.

This gets Slayer running on Haiku again. There are now some layout
problems to solve...