Move management app path
Launch feed managment app from applet

- Added to context menu
- Run management applet in async mode (otherwise we block the whole
  Cinnamon process!) and read the stdout to get new feeds
- Update applet settings with the output of the management applet
Store Feed URLs in hidden applet setting

- A "textview" setting would be preferred, but this is only available in
  Cinnamon >= 2.0.x. Dropping support for Cinnamon 1.8.x is not an
  option yet. For now we will only support managment of feeds via the
  management app
- Add button to settings to launch management app