Patch for topic selector, and dnd for bookmarks menu

[Peter Harvey <peter a harvey gmail com>]

Hi all,

I've been working on a new topic selector for bookmarks. I've posted a
screenshot and patch at:
  http://home.exetel.com.au/harvey/epiphany/t1.png
  http://home.exetel.com.au/harvey/epiphany/patches/t1.patch

The way the new topic selector works is:
 - the left side works just the same as the old topic selector.
 - the right side is a generated list of topics that we think you 
   probably want to pick from, at least to start with

The right side lists (in bookmark count) topics that aren't already
selected, and aren't completely covered by larger topics.

Simple example:
If I have a topic People and a topic Friends, and every bookmark in
Friends is also in People. You could think of Friends as a 'subtopic'.
Initially the list on the right will show 'People' but not 'Friends'.
Once I click on 'People' it is added to my selected topics for the
bookmark. Since we don't list selected topics on the right side, the
topic 'Friends' is now revealed.

Complex example:
If I have topics Bugs, Epiphany, and Evince. It's quite likely that all
bookmarks in Bugs are covered by the combination of Epiphany and Evince.
So my right list shows only Epiphany and Evince. Once I select one of
these, the Bugs topic is revealed.

The end result is a system that *feels* like you're progressively
working down through subtopics, but far more robust for complex topic
topologies. Please try it out if you have the time. There are usability
bugs (eg. you can't use the cursor keys on the right list atm). But I'd
like to get feedback on the look and more importantly the *feel* of it.

Regards,
Peter.

PS. As an added bonus, this patch includes the ability to drag-and-drop
bookmarks and topics from the bookmarks menu to wherever you like (other
applications, the current window, the tab list, the toolbar, etc).