For directories, leave size column blank

Currently in list view, file size column show 4.0KiB for directories, and it causes confusion to users. Although there are some suggestions to solve this issue (like #94, which suggests to show recursive file size and #61, to show number of contained files), none of them made into stable and have potential performance issue.

Easier and faster way to solve this issue would be @suilno 's suggestion#94. If it's a directory, it would be better to leave the file size column blank. Since it's what Windows and Mac do in list view, almost everyone would be familiar with this behavior. At least it would be an improvement on the current situation.

I have no fork permission now, so I would post a patch here: leave_size_column_blank_for_directories.patch