When using multiple area indexes (splitted views) the document switcher
activated with Ctrl+Tab was always changing the document of the first
splitted view even if the second view was selected.

Looking at the plugin code, it uses a list of last enabled views (Sublime::View)
and activate a view thanks to Sublime::Window::activateView, but this function
only show a view in its container (Sublime:Container) which was used when adding
the view.

It tends to confirm that switching a view will always keep it in the same
area index.

To solve this issue, the behaviour of the document plugin (documentview) is
mimicked by using Sublime::[Url]Document instead of Sublime::View and call
openDocument from document controller. This function seems to automatically
detect if creating a new view is needed or a old can be reused depending on
the current area index.

Note: This is my first patch in this projet, please let me know if I miss anything.