Revamp hidden timeout handling
Add a new timeout_style environment variable and a corresponding GRUB_TIMEOUT_STYLE configuration key for grub-mkconfig. This controls hidden-timeout handling more simply than the previous arrangements, and pressing any hotkeys associated with menu entries during the hidden timeout will now boot the corresponding menu entry immediately. GRUB_HIDDEN_TIMEOUT=<non-empty> + GRUB_TIMEOUT=<non-zero> now generates a warning, and if it shows the menu it will do so as if the second timeout were not present. Other combinations are translated into reasonable equivalents.
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