Add BLANKSPACE color

[mrtnvgr]

This PR adds a new color which highlights empty chains in songs and phrase slots in chains.

[djdiskmachine]

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Fixes # (issue)
Type of change

Please delete option that isn't relevant.

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions on how to reproduce.

Test Configuration:

Hardware:
OS version:

Checklist:

I have performed a self-review of my code
I have commented particularly in hard-to-understand areas
I have updated documentation regarding my changes
My changes generate no new warnings

[mrtnvgr]

Type of change: New feature (non-breaking change which adds functionality)
How Has This Been Tested?: the change visuals are correct

Test Configuration:

Hardware: Linux x86, PSP E1000
OS version: NixOS unstable, last PSP OS version

Checklist:

• I have performed a self-review of my code
• I have commented particularly in hard-to-understand areas (there are no hard-to-understand areas)
• I have updated documentation regarding my changes
• My changes generate no new warnings (i'm not sure, don't know how to check this sorry)