Merge activities and timetables in the menu

Currently, three different modes of play are split between two distinct menu options: Activity and Timetable. I believe it would be better merge them together and have a single list containing Explore Route, <activities>, <timetables> and <timetable sets>. We'd keep all the same options but remove the unnecessary choice of two modes (when there's actually three).

The available options would be mixed together and sorted alphabetically, forming the union of the current "Activity" and "Timetable set" lists (the latter contains individual timetables too).

- If you choose Explore Route, you can choose the locomotive, consist, starting at, heading to, time, season and weather.
- If you choose an activity, you can't choose anything.
- If you choose a timetable set, you can choose a timetable, train, season and weather.
- If you choose a timetable, you can choose a train, season and weather.

Discussion: http://www.elvastower.com/forums/index.php?/topic/27968-merge-activities-and-timetables-in-the-menu/
Roadmap: https://trello.com/c/NlJvk8y1/334-merge-activities-and-timetables-in-the-menu