General:
- fixed faulty synchronization, which caused GUI to freeze,
- finally removed that ugly hack for hclock in emulaiton core,
- code cleanup and refactoring
- changed the implementation of frame limiter,
- more progress on connecting sound,
- misc bugfixes.

CPU:
- dummy reads for addressing modes should read Program Counter instead of the effective address,
- fixed pushing wrong values onto the stack by BRK,
- fixed implementations of SHX and SHY,
- other fixes.

PPU:
- partially fixed writes to $2005 and $2006.

Board:
- added mapper 7 (AxROM),
- fixed writing to CHR ROM by PPU.