Adds a progress bar for timetable pre-run

Timetables contain multiple AI trains, any one of which can be pre-selected as the Player's Train. The time between the first train of the timetable and the Player's Train is the "pre-run" which is skipped through at speed.

During the pre-run, the progress bar sticks (about 20% in) and this can be a very long time (minutes).

I propose that any pre-run is done at the end of the loading cycle and a second progress bar is used to show progress only through the pre-run.

Alternatively the existing progress bar could make a second trip across the window.