Add custom Composite Palette file (.tgp) support

[pceDev16]

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Add for a means to load custom composite Palette files, in the MiSTer .tgp format, so the user can add in some other ones besides the default.

Custom Palettes can be created with the tool in this repo: https://github.com/Kitrinx/TG16_Palette

If there is interest I can provide a sample custom palette to test with as well that mimics the appearance of a TG16 connected over RF to a CRT!