version 6.0 beta

Big new release with many improvements and changes (some backwards-incompatible). Hence a new major version number 6.

This is a BETA release and as such things may still change.
I'll write better release notes for the final 6.0 version once it's done.

The most important improvements and changes are:

• restructured part of the library modules, may break old code
• moved many built in functions that should just be subroutines, to the library module "sys" or "string", may break old code
• commanderX16 virtual registers cx16.r0 .. r15 available (for both cx16 and c64 compiler targets)
• added new library modules and routines such as gfx2 and new diskio stuff