Added custom-field copy to the context menu on entries. It's not fully functional as it's not correctly using the click but always the selected entry!

MacPass/MPEntryViewController.h

@@ -48,9 +48,11 @@ typedef NS_ENUM( NSUInteger, MPCopyContentTypeTag) {
 /* Copy/Paste */
 - (void)copyUsername:(id)sender;
 - (void)copyPassword:(id)sender;
+- (void)copyCustomField:(id)sender;
 - (void)copyURL:(id)sender;
 - (void)openURL:(id)sender;
 
+
 /* Entry Handling*/
 - (void)deleteNode:(id)sender;