Example-based grammar writing: fix problems caused by the use of exprToAPI

The editor needs to keep track of both the raw term and the nice term returned
by exprToAPI. (Manually constructed linearization rules will now have the
raw term and can not be tested.)

Also replace metavariables in generalized terms with the apropriate parameter
from the linearization rule.

Also fix communication problems caused by inconsistent use of show/read vs
showExpr/readExpr.

src/example-based/ExampleDemo.hs

@@ -1,4 +1,4 @@
-module ExampleDemo (Environ,initial,getNext, provideExample, testThis,mkFuncWithArg,searchGoodTree)
+module ExampleDemo (Environ,initial,getNext, provideExample, testThis,mkFuncWithArg,searchGoodTree,isMeta)
   where
 
 import PGF