Experimenting with conveyor parts

I added 2 experimental conveyor parts

• one with 2 borders - a bit easier to handle and
• one with no borders at all - this seems to me way too challenging and annoying

I also added a config screen that comes up before the game starts. You can select these new conveyor parts before starting the game - either select only one type or even both.

The config is not saved. This is just for easier experimentation. And, of course, all the levels totally change with different setting.

This is Level 4 with only one type selected: