General: code cleanup and refactoring.

GUI:
- further work on connecting audio to GUI;
- started reimplementing NES 2.0 header.

Board:
- fixed CHR bankswitching for MMC3;
- added mapper 88 (Namco-3433/3443);
- added mapper 95 (Namco-3425);
- added mapper 154 (Namco-3453);
- added mapper 206 (Namco 118).