Added support for URL management via file.

    I added a second method to manage the feeds using a file.
    There is a new settings field specifing a path to a file. The user can then
    add one feed url per line.
    A second new settings field is used to switch between the two methods.
    I also added a new context menu entry which reloads the file.
Changed feed file settings type to a filechooser.

    The feed file can now be selected via a filechooser.
    The applet also does not crash anymore, if the file is not
    readable, but displays an error message in the popup menu.
Settings file chooser now depends on checkbox.

    The settings file chooser is now only active if the file is
    selected as source via checkbox.
Bump version to 0.7