[feature] Added a system event to allow developers to assign Smarty variables

### What does it do ?

Adds a system event to enable developers to assigner Smarty variables to be used in user navigation (ie. to display a notification count number on a menu entry)

### To do before merge

- [ ] triple check it is not possible to override critical variables (ie. `$_config`)
- [ ] make sure Smarty variables are usable/parsed in lexicon strings