Upgrade SDL from 1.2 to 2.0

[tomangelo]

As it was mentioned in https://github.com/colobot/colobot/issues/408, current Mac versions of Colobot may not have icon, because SDL1.2 wasn't updated on this OS. In fact SDL 1.2 may be problematic in future development, especially if we'll think about porting the game to another platforms. A successor, which is SDL 2.0 has many new features https://wiki.libsdl.org/MigrationGuide, that may be useful in future releases. Another advantage is that it's still supported, so eventual bugs will be fixed.
We should think about sending worn-out SDL1.2 to retirement and replace it with new, up-to-date SDL 2.0. I've looked at our milestones https://github.com/colobot/colobot/milestones/ and I think that alpha-0.1.6 would be good for this, but if someone have an another opinion then he may propose another date.