This makes checking for the primary file easier, without having to know
the index of a file in the list of tabs, or relying on the fact that the
primary file is always first (it still is, though).
This changes some places in Sketch to use the new
`SketchCode.isPrimary()` method, but there probably are a lot more
places in the code that could be start to use it as well.
It was not used, and since it only updated the `name` attribute, but not
the corresponding `file` attribute, nor actually handled renaming actual
files, having this method around would actually be harmful, so just drop
it.
For completeness a sketch should allow the same extensions as a
library. These missing sketch extensions have already been enabled for
libraries in #3186.
