General:
- code cleanup and refactoring,
- misc bugfixes.

CPU:
- replaced current addressing mode detection with something much more reliable (especially for unofficial opcodes),
- added more info for debugging logs.

Boards:
- fixed masking for UxROM.java

APU:
- fixed unnecessary DMC DMA cycle stealing,
- started connecting audio to GUI.

GUI:
- now at least a background shows up! yay for progress,
- started working on saving preferences,
- connected joypads to GUI.