#162 Icon shows up even when -n option is given

[Ian]

This is with a generic system tray (tint2).

1. Start parcellite as "parcellite -n". No icon so far.
2. Hit ctrl-alt-P, open the preferences dialog, and change the icon name. Click OK. Icon appears. Could say we were begging for it by messing with the icon preference, but seems like the command line should overrule anything that happens in the preferences,

setup_icon() is called from check_sanity() without checking the flag for whether to show the icon or not. Only happens if the icon name changed, looks like it can leak memory if you change the icon name more than once.