Add subtitle preview

[jonko0493]

We should expand on #187 and add a preview for subtitles to make it easier to see where they'll show up on the screen. All the information for drawing them is contained in the subtitle structs, so it shouldn't be difficult to implement this.

• The struct contains a top/bottom screen enum which can be used to determine which screen to draw to
• The struct contains X/Y coordinates for drawing position on the screen
• The actual drawing routine is as follows:
  • On the top screen, the subtitle is simply drawn in white text at (X,Y)
  • On the bottom screen, the subtitle is drawn in white at (X,Y) and then drawn in black at (X+1,Y+1) and (X,Y+1) behind the white text
• Ensure you use .Subtitle so that the formatting (#P07) is stripped