v2.6

This release is the first of several that mainly contain changes to the internal architecture. My longer-term goal is to port the new emulator architecture from VirtualC64 v5.0 to vAmiga, eventually enabling neat features like run-ahead. Since the architectural changes are considerable, several intermediate releases will be part of this project. The most noticeable changes of this release are related to RetroShell, as the syntax of some commands has changed.