Add support to import OPML rss feed list

- Add menu item to feed management app to import OPML feed list
- Fixed #25
Feed management app unicode support

- Support reading unicode from OPML files
- Support reading UTF8/ASCII from input file
- Support reading UTF8/ASCII from stdin
- Support output in utf8
Make feed management app feed list scrollable

- Stops the window becoming huge if a large number of feeds are loaded
  or the feed URL/title is very long