Option to easily navigate between files in non-tabbed view

[gusthoff]

• In non-tabbed view, pressing the down key in the last line of a file could allow to switch directly to the next file (next pane in the editor).
  • Of course, as expected, pressing the return key instead should create a new line in the same file (same pane).
• Because not everyone might like this, it could be implemented as an optional feature.
• This suggestion is related to the changes for Typescript webpack refactor #274.