File widget: use SDL_ShowOpenFileDialog() when SDL backend is active

[RobLoach]

When the file widget is built against the SDL backend (nuklear_console_file_system.h), file browsing today still goes through the in-widget list view. SDL3 ships a native picker API — SDL_ShowOpenFileDialog() (plus SDL_ShowSaveFileDialog() and SDL_ShowOpenFolderDialog()) — that hands off to the platform's real file chooser. Using it would feel more native on Linux/macOS/Windows and would also resolve the keyboard/gamepad navigation edge cases the in-widget browser carries.

Refs:

• https://wiki.libsdl.org/SDL3/SDL_ShowOpenFileDialog
• https://wiki.libsdl.org/SDL3/SDL_ShowSaveFileDialog
• https://wiki.libsdl.org/SDL3/SDL_ShowOpenFolderDialog

Suggested Implementation

• Add an opt-in macro NK_CONSOLE_FILE_SDL_NATIVE_DIALOG (default off to preserve current behavior) gated by NK_CONSOLE_ENABLE_SDL3 / equivalent.
• When set, nk_console_file / nk_console_dir / nk_console_file_action skip the in-widget list view; on activation, they call SDL_ShowOpenFileDialog (or SDL_ShowOpenFolderDialog for dir, SDL_ShowSaveFileDialog for the save-action variant).
• Wire the SDL_DialogFileCallback to copy the chosen path into the existing file-path buffer and fire NK_CONSOLE_EVENT_CHANGED.
• Route any nk_console_file_set_filter extensions (File: Extension Filter #175) through SDL_DialogFileFilter[] so the native dialog honors the same filter spec.
• Document the new macro in the README Configuration table.

QA

• demo/sdl3_renderer with the macro enabled opens the platform's native file picker on click.
• With the macro unset, the existing in-widget browser still works (no behavior change for current users).
• Save-action variant returns a path even when the chosen filename doesn't yet exist on disk.