added base folder EmptyTrash() method

mailnews/base/util/nsMsgFolder.cpp

@@ -635,7 +635,12 @@ NS_IMETHODIMP nsMsgFolder::CreateSubfolder(const char *folderName)
 
 NS_IMETHODIMP nsMsgFolder::Compact()
 {
-  return NS_OK;
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP nsMsgFolder::EmptyTrash()
+{
+    return NS_ERROR_NOT_IMPLEMENTED;
 }
 
 NS_IMETHODIMP nsMsgFolder::Rename(const char *name)

mailnews/base/util/nsMsgFolder.h

@@ -137,6 +137,7 @@ public:
   NS_IMETHOD CreateSubfolder(const char *folderName);
 
   NS_IMETHOD Compact();
+  NS_IMETHOD EmptyTrash();
 
   NS_IMETHOD Rename(const char *name);
   NS_IMETHOD Adopt(nsIMsgFolder *srcFolder, PRUint32*);