Scenario Chooser and Loader Now Working

The Scenario Browser will now let you select a scenario from a list of those in /data/scenarios, display the selected scenario in the Lobby menu, and run the game with the chosen scenario's map (or the default map, if none is indicated in the scenario file).

Before this feature is really 'complete', there are a few more tasks:
# Starting with the default scenario as chosen in the Lobby.
# Saving a scenario in the editor also creates a .sce file (at the moment it just saves a map)
# Properly implement the scenario loader in the scenario editor (and give the browser exclusive mouse control)
# Sending the chosen scenario and map to the client either to a single client by client request (on connect) or upon change - to all the clients.
# Client responds if it doesn't have the same version of the map/scenario or doesn't have it at all
"You do not have the chosen scenario, and the server is not allowing you to download it directly"
# Testing the system on a Windows machine and making any adjustments
# Proper error messages if files can not be opened etc.

This feature will also be extended:
# Building a file saving version of the browser, with "overwrite, yes/no" message box widget
# Allowing the scenario editor to modify .sce details before saving (possibly a separate feature)
# A more robust file loader/finder (not really essential)