Add split in new window action for editor views.

A few details are still to be done:
- extra windows are not restored yet
- window title of extra windows should show information about current
  editor
- the "raise main window" hack for Linux doesn't work with Qt 5

But it should already now be quite usable.

Change-Id: I638583ee4b2abe32e95c261bdf31b4a10b472748
Reviewed-by: David Schulz <david.schulz@digia.com>