track labels

track labels
ticket:3
This adds two side bars to the PathWidget that show the current position
(and the total length of the sequence, and should show the name).

Scrolling the viewport around makes the position number change too.
(Which revealed a bug that glsequence.leftbase would return values larger
than the sequence).

I also fixed the default size hint.

However all is not completely well, I really need to specialize the
right and left sidebars. And there were way too many classes created for
trying to handle all the user interface widgets. And I'm the locations
of the various signals and updating code could certainly be better thought
out.