Bug hunt

Added box zoom with indicator window.  
Re-fixed the bad trace number checker by putting the loading of the sdi depth line (current surface) back into the surveyline load function so that the bad trace nums can be corrected before the line is made.  This should eventually be done in the sdi.binary module.  
fixed the cursor freeze key to work on alt-c so that it doesn't interfere with the alt-f on some systems.

next major task before adding features is to organize the depth line creation to be more robust.  Currently it can end up in a bad state if things are done out of order and changing attributes wont work correctly.