[sldev] Shortcut keys for rendering features

[Argent Stonecutter]

The biggest problem I have right now in terms of shortcuts is that ^N  
isn't a global toggle. If they're not going back to the old alt-shows- 
physical behavior then they should at least make ^N toggle beacons in  
edit mode too.

This is kinda related to the "scriptable UI" stuff. We really need a  
set of keys_$MODE.xml files that let us override any key binding in  
any mode.