Instead of manually sorting the primary file at the start, and fiddling
to keep it there during resorting, this just modifies the sorting
comparator used to sort any primary files at the start. This is slightly
more generic than needed, also supporting multiple primary files, to at
least not break the Comparator preconditions when for some reason there
are multiple primary files.
These used to iterate over the list of SketchCodes to find the right
one, and if so, let the List do the same again to remove it or find the
index. This can be simplified to just let list take care of things
instead.
Technically, there is a small difference, since `List.remove()` and
`List.indexOf()` will check using `equals()`, while the original code
used `==`, but these should be effectively the same here. Also, the
original code first used `==` to see if the object was present and then
let List find it again using `equals()`, so that was a bit inconsistent
anyway.
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.
