Refactor to use view name instead of view id

This was is partly from the original code review round. At that time
Brecht Van Lommel suggested using names for the API instead of integers
(as we do for RenderLayers). I went a bit further and changed *most* of
the code to use strings.

The only parts that still use view_id are the ones closely related to UserImage.

That simplify the code and should make the code easier to understand,
review and maintain.

I tested many (if not all) of the supported areas, and apparently things
are working. But given the size of the intervention I wouldn't be
surprised if something stopped working, so please report.
Filepath should include view even when rendering one view (if multiview is on)

 When multiview is not used the filepath is as usual (e.g., Image.jpg).
 When multiview is on, even if only one view is enabled the view is incorporated
 into the file name (e.g., Image_L.jpg). That allows for the user to re-render
 individual images.