Improvement for the note list screen

Hello @dotemacs

Here is one more request I'd like you to merge.

In the note list screen, each header line is supposed to be aligned to the length specified by _simplenote-note-head-size_. But it doesn't work as expected for the multi-byte characters like Japanese. That's because the function _length_ doesn't return the actual column size of string. I fixed that by using _string-width_ instead of _length_.

Also, I made _simplenote-note-head-size_ customizable and added a new customizable variable _simplenote-show-note-file-name_ so you can hide the file name of each note, because I think it's useless information for most users.