Detect XML data starting with a declaration

Checking for "<?xml" at the beginning of a text is a quick and simple test
that will detect most of the XML cases.

This fills an obvious gap in #1537.

src/EditDialog.h

@@ -74,7 +74,8 @@ private:
         Null,
         Text,
         JSON,
-        SVG
+        SVG,
+        XML
     };
 
     // Edit modes and editor stack (this must be aligned with the UI)