systemActions: Use 'system-log-out-symbolic' instead of 'application-exit-symbolic' for logout

Changing the icon to 'system-log-out-symbolic' has no visual change in a default GNOME setup since both 'system-log-out-symbolic' and 'application-exit-symbolic' are the same in adwaita-icon-theme (at the time of writing), however, other icon themes differentiate between the two icons so pointing to the appropriate icon name is the right thing to do.

Closes #2751 (closed)