Feature: Optional per-class backgrounds for custom themes

[DanGM96]

Extends the patched gfxmenu so a theme can change backgrounds depending on the highlighted menu entry’s classes, similar to how it's done for icons/.

How it works:

• If the theme directory contains a backgrounds/ folder, the feature is enabled
• For the selected entry, GRUB walks entry->classes and loads the first match: backgrounds/<class_name>.png
• If no matching PNG exists for that entry, the view falls back to the default image (theme.txt desktop-image).

Notes:

• If there is no backgrounds/ folder, it behaves the same as expected without this feature
• When doing full redraw, you can notice a small delay
• I tried optimizing it without going too far out of scope, but the line count still ended up bigger than I expected

How to add to themes

• Simply add a backgrounds/ directory inside the theme file (same layout idea as icons/).
• Create PNGs named after class strings (e.g. same classes as in menu_class / GRUB class on entries).

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Inspired by https://github.com/elwint/grub-gfxmenu-bg-mod