Demo update v.0.7.3

Demo update v0.7.3

New key bindings! WASD and the middle mouse button are now used for map navigation.

Other changes:

• fixed the train insertion icon in the train assembly window;

• fixed trains disappearing beyond the edge of the screen (they now return to the depot);

• fixed train behavior when using only a single locomotive;

• hid a secret sign;

• removed UI blocking by dialog windows (it was annoying);

• removed the ability to mirror buildings;

• fixed a crash when attempting to add a wagon to a train that was already on the tracks;

• made minor train behavior improvements: the train's front now detects signs that were placed after the train had already been put on the tracks;

• train camera improvements were postponed because @metalcoverua still hasn't given a masterclass on how to flip trains properly.