Sort out tabbing order in the app

One thing that didn't work 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.
EOL at EOF
Merge pull request #2097 from ussuri/sort-out-tabbing-order

Sort out the tabbing order in the UI