It is undefined behavior to create a std::string from NULL. This
adds checks to prevent accidental comparison or assignment with
NULL to the String setting class.

When switching modes between iconview and treeview, it would skip
the first treeview item when the down arrow was pressed. This
properly resets the selection without interfering with the search
results selection.

Thunar now checks for a trusted bit set for launchers copied to
the desktop folder. To avoid making the user deal with a prompt
after they add a launcher to the desktop, use Thunar to copy it
so that Thunar will set the trusted bit.