Sort out the tabbing order in the UI

TBR @dinhviethoa, particularly for the changes in tabview.dart.

One thing that hasn't worked out is completely preventing the editor from receiving the focus while tabbing through the elements of the UI. Set the top-level container's tabindex to -1, but the contained Ace's elements don't have tabindex set, so after making a round over search box, Run, menu, Back, Forward and file tree view, the editor still receives the focus.