This patch contains a few things:

* Visual improvements to the sequence strips. The look has been modernised as a nice companion to Peter's new modernised backend, whilst also improving visual communication. The background alternating bars are based on code by Campbell Barton. Sequence strip colours are now customisable as theme colours.

screenshot: http://mke3.net/blender/interface/seq/seqstrips.png

* Refactor and cleanup of the sequence strip drawing code in the process.

* In image preview mode, the image can now be panned by dragging it with the middle mouse button. Home key resets the position and zoom back to default.