Flexbox motto: columns have width; rows have height.

Put QPOutput up on level and wrap all of the
interior boxes in .hbox
Use the width of the containing hbox for computing
the offset of the token hightlight
Some design pages to clarify flexbox model.
One looks similar to qp.

One has a similar layout modifed from online demo.

Another show how flex box works with percentage widths.
Together with the previous three commits, fixes #106,
Editor does not resize correctly when the window is expanded
then reduced in size.
Setting the min-width is critical for our flexbox layout