Conversion of SimpleTimeline to Goocanvas almost complete

I solved my conceptual dillema by introducing Yet Another Layer of Abstraction: I subclassed HList() to create TimelineList(), which harbors all the application-specific code. This required reworking the HList() class a bit, but it still works as it did before. I even manged to get rid of some code I didn't like in the process. So, now the mouse events trigger changes to the underlying composition, and the UI is only updated in the callback. All that remains now is to handle the external drag-and-drop events (of FileFactories onto the timeline) a little better, so that the user can see where their source will end up before making the drop.

I asked about my pointer-event propagation problem on the goocanvas developer mailing list. It turns out that the only way to get signals from goocanvas.Widget items that contain a widget with an xwindow is to connect to the widget itself, rather than the goocanvas.Widget item. This will require conversion of the coordinates. That might not be so hard, so it's worth pursuing. The current "ghostly" SimpleSourceWidgets are rather distracting, but it's low priority for now

After that, I'm going to start porting the advanced timeline over to goocanvas. This should be relatively straightforward. Each source in the timeline will have a goocanvas.Rect() object, or perhaps a derivative, which will represent it in the timeline. There will be a direct mapping from time to position along the x axis. Zooming will be handled by adjusting the scale_x property of the canvas, and scrolling will be handled automatically by a gtk.ScrolledWindow (that will also contain the SimpleTimeline, and handle scrolling for that as well). Once that's done, PiTiVi will basically work as it did before, except that now it will be easier to add new features using goocanvas. By this time I hope to have development versions of gstreamer built, because I'll be on my way to europe.