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.

[piotrdz]

I think it's a good plan. Migration itself does not require much effort and we should make it to 0.1.6 without problem. I think until this time also all major Linux distributions will have SDL 2.0 packages available as well. We should remember that upgrading in our case means changing three libraries: SDL, SDL_image and SDL_ttf.

[RaptorParkowsky]

Well, if this will be compatible with Windows 2000/XP and most of the graphic cards from 2005 (for older models and textures) and from 2008+ (for newer models and textures), then I also 100% ok with that update : ) .

[tomangelo]

I've chose 0.1.6, because all next milestones include refactoring of some code, and 0.1.6 doesn't, it have only graphic issues, so I thought that this release could have some bigger code change too. Especially if this would make it easier to make new features in a future.
@Raptor: As far as I know, the graphic is mostly written in OpenGL itself, SDL1.2 hasn't so good hardware acceleration, so upgrading this library shouldn't cause any bigger trouble with old hardware. But still I could be wrong, if someone know why this could make any trouble, then he would inform us.