Adding the prototype GF editor for simple multilingual grammars

src/editor/simple/TODO

@@ -0,0 +1,28 @@
++ Safety question before deleting a grammar
++ Check identifier syntax
++ Allow lincat for deleted cat to be deleted
++ Allow lin for deleted fun to be deleted
++ Apply category alpha conversion in concrete syntax
++ Remove a concrete syntax
++ Apply function alpha conversion in concrete syntax
++ Change lhs of lins when function type is changed
+
+- Allow languages other than the ones in the given list to be added
++ Export as plain text
+- Allow definitions to be reordered
+
++ 1. possibility to compile the grammar set, returning a URL to a translator app
+- 2. possibility to import modules - both resource libraries and user's own
+     auxiliaries
+- 3. possibility to upload own modules
++ 4. access to the created files in an on-line shell (making testing possible)
+- 5. rule-to-rule type checking and guidance (e.g. with library oper
+     suggestions)
+-    Try grammars in the Translation Quiz
+
++ compile only the uploaded grammar even if other grammars are present
++ 'flags startcat' is needed for grammars with only one category (since the
+   default startcat is S, even if it doesn't exist)
+
+- Bug! After adding a 2nd def of a fun with a different type and then deleting
+  the old fun, the corresponding lin will have the wrong lhs.