Progress!

Masked error caused by saving dbc while editor is open that causes
msg IDs to go crazy by making editor modal, but that affects all
other windows, so might be a better fix.

Added new value type of VALUE_STRING, probably better named
ENUMERATED_STRING, and it uses the table in the editor to define
the enum.

Fixed a bug in the signal viewer load when a signal is not matched
(I found it by renaming a signal and then loading)

dbc/dbc_classes.h

@@ -21,7 +21,8 @@ enum DBC_SIG_VAL_TYPE
     SIGNED_INT,
     SP_FLOAT,
     DP_FLOAT,
-    STRING
+    STRING,
+    VALUE_STRING
 };
 
 enum DBC_ATTRIBUTE_VAL_TYPE