shortcut changed in Preferences are saved between sessions

ChangeLog

 Version XX
+	Shortcut changes made in the 'Preferences' menu are saved between sessions (the 
+	shortcut_cache flag in the options file can be used to turn this on/off).
+	
 	More support for GTF format to show CDS and exons as joined features.
 
 	BAM record list option added to display as a list the reads and their