Feature: Custom dialog option highlight colour

Allows the user to replace the default yellow for dialog options with a colour of their choice. For text window GUIs, the highlight colour remains determined by the text colour of the text window GUI. This retains compatibility with earlier versions.

I was hesitant to call dialog_highlight_colour because it is not effective for text window GUIs, but I'm fairly certain we could explain the exception or have a swtich (game.use_text_window_colour_for_highlight) down the track if it's really necessary.