The System Trace isn't preserved when quitting Rockbox Utility. Change this so
the last trace is saved in the cache folder, and add a button to the trace
window to allow easy saving it. Should help in cases where users have problems
but restart Rockbox Utility before saving the trace.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@25313 a1c6a512-1295-4272-9138-f99709370657
