Oh man, this is really great

Lot of cool things here I suddenly wish I'd implemented in the VVVVVV editor back in the day!
I love the brush size settings. Full horizontal/vertical is a really nice addition!
The scripting stuff seems really well implemented: looks like it's pretty nice to move between scripts, and the syntax highlighting is great.
(Found a small crash bug, btw - selecting "edit script" from a right click menu on either a scriptbox or terminal causes it)
One of my biggest regrets is that VVVVVV didn't have built in instructions on how to use the editor, especailly for scripting. So nice work in particular on that
