Experimenting with skydome settings (#5)

[codecat]

I'm not sure how to implement this properly, but I pushed the code for it anyway. In Tweaker, this used to have an "Apply" button. It's a little inconsistent and not very useful if you have to press this button every time. So this should, at minimum:

• Automatically update when loading into a map
• Reset to the original mesh instead (eg. Canyon mountains) of picking either a double dome or a full dome
• Have shortcuts and menu controls

The first 2 points are obviously the most important. Input welcome.