Code refactor and bug fixes related to bgm.

[Wolf3s]

Pull Request checklist

Note: these are not necessarily requirements

• I reformatted the code with clang-format
• I checked to make sure my submission worked
• I am the author of submission or have permission from the original author
• Requires update of the PS2SDK or other dependencies
• Others (please specify below)

Pull Request description

This cleanups and refactors opl as whole.
Changes being made so far:

• Removed redudant includes
• Refactored menu.c and audio.c
• Moved all embedded files to the newly imports.h(extern-irx.h)
• Fix game launching when bgm is activated.

TBD:

• Split opl.c
• Cleanup ee_core
• Refactor all the modules

Size:

Master branch build:
1.337.044 bytes
Cleanup branch build:
1.336.436 bytes

NOTE: There´s a branch that i will push some experiments and with time. I perhaps merge.
Happy new year for all!!!