Hotkeys for Part; Ation group; and Crew editing modes

[CaptainTurbomuffin]

While making spaceplanes, you have to switch a lot between action group editor, and part placement modes. Some hotkeys would be helpful.

Also, I still encounter the bug where an action group is only controlling one of two engines, while the blue selection suggests it controls both of them. It's a pain in the neck, something has to be done on this.