View: Adjust top margin when needed

When the view is big and the traces are vertically centered,
there will be blank space above and below the block of traces.
When the user then resizes the window and makes it smaller,
the white space stays, pushing traces out of the view that
could fit on-screen if the blank space was adjusted properly.

This is what we do with this patch.

If there is still enough space to make everything fit, we
center the traces vertically. If there's not enough space
to make everything fit, we make sure that the top traces
are shown.