Add ncurses-based interactive theme selector (like `kitty +kitten themes`) for xfce4-terminal

Description: It would be very useful to have a built-in, interactive way to browse and switch color themes in xfce4-terminal, similar to the kitty +kitten themes command in the Kitty terminal emulator.

Feature details:

• Provide an ncurses-based menu that can be launched from the terminal, allowing users to preview and select from available color schemes for xfce4-terminal.
• The selected color scheme should be applied instantly, without the need to manually edit configuration files.
• Optionally, provide a command-line option (e.g. xfce4-terminal --themes) to open this theme selector.

Benefits:

• Users can easily try out and switch between color schemes directly from the terminal interface.
• No need to manually edit configuration files or restart the terminal.
• Improves usability for users who prefer working in the terminal.
• Brings feature parity with modern terminal emulators like Kitty.

Thank you for considering this enhancement!