Give the rebase interactive view some UI love

Give the rebase interactive view some UI love

* Use shorter labels for the up/down ToolItems: GTK scales all buttons
  to the same size, making the other buttons appear way too wide if a
  long label is used, and thus the whole toolbar required lots of
  horizontal space.
* Pack the toolbars. On GTK, this often (though not always!) fixes the
  mis-alignment of some labels, which appeared left-aligned instead of
  centered.
* Remove a useless SashForm with only one child, and remove some more
  dead UI code.
* Move both toolbars into the form header, one left-, the other right
  aligned. Remove the now pointless form section.
* Remove the background from the toolbars so the form header's gradient
  shows through.
* Calculate the width of the "Commit Id" column based on string extents
  to avoid the column header gets clipped.

The net result is a much sleaker and much more compact view that has
more space for the actual plan items. See the before-after screenshots
attached to the bug report.

Bug: 498014
Change-Id: Iac7abd41567e8206e506b2858722534e604b67c0
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>