Minor tweaks to shadow warning infrastructure

1. Moved some methods out of boot.js because they are not needed until
after bootup
2. Added alternate message for editing an overridden shadow tiddler
3. Minor style tweaks

core/modules/tiddler.js

@@ -20,6 +20,10 @@ exports.isPlugin = function() {
 	return this.fields.type === "application/json" && this.hasField("plugin-type");
 }
 
+exports.isDraft = function() {
+	return this.hasField("draft.of");
+};
+
 exports.getFieldString = function(field) {
 	var value = this.fields[field];
 	// Check for a missing field