Correctly read files that include a 'replace' directive

- Closes simos#23
- This commit only fixes the Exception occurring when reading a file
  that contains a 'replace' directive. However it does nothing to
  actually process it!
  Therefore another commit is necessary to actually handle 'replace' (in
  addition to 'override').

XKBGrammar.g

@@ -38,6 +38,7 @@ tokens
   ELEM_VIRTUALMODS;
   KEYELEMENTS;
   OVERRIDE;
+  REPLACE;
   OVERLAY;
 }
 
@@ -97,7 +98,7 @@ line_keytype
   ;
 
 line_key
-  : OVERRIDE? 'key' '<' NAME '>' '{' keyelements (',' keyelements)* '}'
+  : (OVERRIDE|REPLACE)? 'key' '<' NAME '>' '{' keyelements (',' keyelements)* '}'
   -> ^(TOKEN_KEY OVERRIDE? ^(KEYCODEX NAME) keyelements+)
   ;
 
@@ -120,6 +121,10 @@ override
   : 'override'
   ;
 
+replace
+  : 'replace'
+  ;
+
 keyelements
   : elem_keysyms 
   | elem_keysymgroup
@@ -174,6 +179,10 @@ OVERRIDE
   : 'override'
   ;
 
+REPLACE
+  : 'replace'
+  ;
+
 NAME
   : ( 'a'..'z' | 'A'..'Z' | '_' | '0'..'9' | '+' | '-' )*
         ;