[#20] Implement imrpot/export menu
[#20] Implement export from sidebar. The exported stickies json file name is "stickynotes_(all|page_:page_id|tag_:tag_id).json"
Refactoring newline characters of sidebar.xul: crlf -> lf
Refactoring: use this instead of stickynotes.Sidebar in Sidebar.js
Refactoring: fix coding style