Tweaks to the hull customiser

Absolutely love GSB2 so far, excellent job Cliff!

I have noticed a bug in the hull customiser - mostly irrelevant, but still present - if you alt-tab out while you’re in the customiser, then go back in, the ship will disappear when you attempt to click on it or modify it.

Secondly, it’d be nice to be allowed to change the z value of the base hull images - I have a few ship ideas for engines underneath the hull, as opposed to above.

Yikes. that first one is definitely one I should fix!
The z-order of the hull thing is something that clearly should be editable, although I have a horrible feeling their are massive implications to allowing this, due to all kinds of code assumptions about the hull being on the bottom. I agree it would open up a lot of flexibility though.