Category Tab Bar, Textual Newsfeeds

Many changes:

* Newsfeeds are now managed by category.  Categories are displayed
on a flickable tab bar at the top of the screen.

* Text-based newsfeeds are now supported (in a limited manner).  A
new window containing a QWebView widget is used to display them.

* Audio and video player windows now include the ability to "seek"
to a particular location in the media file.