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
Your pull request still contains merge conflicts. You'd better fix them first ;)