Apsis icon and crosshair polish

I've made some polishing to the apsis icons and the crosshairs.
Crosshairs have bigger graphics, so they appear crisper (they are scaled), and they have a dot in the center now.
![screenshot-20141019-123959](https://cloud.githubusercontent.com/assets/4182678/4692834/71736876-577c-11e4-81a5-97780a297a54.png)

The apoapsis icon is now just an outline triangle pointing up, and the periapsis icon is a solid white triangle pointing down. They are higher resolution, but have some padding, so they appear smaller (it's a bit hacky, I have to admit). I think they are quite obtrusive and distracting at a larger size.
![screenshot-20141019-124051](https://cloud.githubusercontent.com/assets/4182678/4692835/73de42fc-577c-11e4-9615-c773c92bd4d5.png)