Call helper

Another round for finding calls to sub-actions.

- My other search missed some variants like $sub_action and $subaction thus a few other places were still using the string directly fixes #1895  /me hopes...
- call_helper() will now return a boolean false/log an error if the given string cannot produce a callable var.
- Removes some is_callable() checks and instead it places an !empty()
Shouldn't this be singular?