Debugger improved: Register editor added

How to use the register editor:
1. Load an .ELF file
2. Select any instruction from a thread. (This is a workaround to
activate the appropriate event listener. This will be changed in the
future).
3. Press 'R' key and modify any register you want.

Note:
The register editor only works with PPU and SPU threads.

Additional changes:
* Fixed the filesize problem caused by the instruction editor dialog.
* Instruction Editor: Fixed small issue in SPU threads