BackendModel::insertExtra() added

## Why?
This function was missing from BackendModel (which already contains getters, updaters and deleters).
It allows somebody to easily add widgets/blocks/homepage-items to extend the websites functionality.

## New stuff:
* Core: BackendModel::insertExtra() added
* ContentBlocks: now uses BackendModel::insertExtra()
* Faq: now uses BackendModel::insertExtra()
* FormBuilder: now uses BackendModel::insertExtra()
* Location: now uses BackendModel::insertExtra()

Fork CMS version 3.7 applied to previous PR https://github.com/forkcms/forkcms/pull/637
Improvement for https://github.com/forkcms/forkcms/pull/725
Comments on code can be useful when they explain why something was done. Just explaining what the syntax does is just overhead and that clutters your function.

Can you please cleanup the comments a little?