Implement accuracy-level-3 Blitter

[dirkwhoffmann]

Right now, three Blitter accuracy levels are supported:

• Level 0: Fast Blitter
• Level 1: Fake Slow Blitter (Fast Blitter + Accurate bus cycle allocation, mainly used for debugging)
• Level 2: Micro-coded cycle-accurate Blitter

The Trackmo demo discussed in #641 is not compatible with either of them. It (accidentally) writes the Blitter registers while the Blitter is running. The new level 3 Blitter should emulate the Blitter hardware according to the Agnus schematics. It should be capable of handling on-the-fly register modifications.

[dirkwhoffmann]

#708 discusses another demo which relies on accurate Blitter emulation.