gfse: you can now add concrete syntax for arbitrary languages

Before, you had to pick a language from a list of supported languages, but now
you can also pick Other and change the language code afterwards.

src/editor/simple/TODO

@@ -22,6 +22,8 @@
 +    Show lincat and lin before params and opers below
 +    Create a new concrete syntax by copying an existing one.
 -    Easy access to compute_concrete from the editor
+-    Instead of starting with an empty grammar, start a new grammar by copying
+     an example.
 
 + compile only the uploaded grammar even if other grammars are present
 + 'flags startcat' is needed for grammars with only one category (since the