You can put down colision lines, and select them (hover the mouse over them), and they turn white. But that's about it. I did my optimization too; it won't try to render or test for mouse intersections with collision lines that are offscreen. And I added a little bit more optimization, so when you draw lines, if the editor finds two lines that are less than 7.5? apart (I.e. you just made an almost straight line from two shorter segments), then it'll take out the middle vertex, and give you one longer line.
I still need to add the ability to copy, paste and delete lines, and they still need to be loaded and saved with the map file. I expect to get the copy, paste and delete done tomorrow, and perhaps the serialization. It just depends if I'm in the mood or not.
Here's a slightly better screenshot, showing the mouse over the dot under the exclamation mark: