gui: map evdev keycodes to HID directly, bypassing host keyboard layout

[SolAstrius]

Both the Wayland and X11 backends previously delivered HID usage codes based on the keysym for each physical key. xkb_state_key_get_one_sym() (Wayland) and XGetKeyboardMapping()'s slot 0 (X11) both apply the user's loaded keyboard layout, so a host running Workman / Dvorak / Colemak / AZERTY would feed the emulator layout-translated letters rather than physical key positions.

For an emulator pretending to be hardware with its own keyboard — or its own layout assumptions, e.g. the ZX Spectrum expects USB-HID-style positions for its 8×5 matrix lookup — that's the wrong abstraction. Layout translation should happen inside the emulated machine, not on the host side.

This routes both backends through a direct evdev-keycode → HID usage table:

• Wayland: wl_keyboard.key delivers key as the evdev keycode. Map directly via wayland_evdev_keycode_to_hid().
• X11: xkey.keycode is evdev+8 on Linux X11. Subtract 8 and look up in the same table. Falls through to the existing xkb-keysym path for non-Linux X11 servers (where the offset isn't 8) or for keycodes outside the standard evdev range.

Result: the emulator sees the physical key the user pressed, regardless of host keyboard layout. On Wayland the keysym path is now fully unreachable, so it's removed; on X11 it's retained as the non-Linux fallback.