Allow modifying operation on Model (e.g. push, insert, remove) #9412

It would be useful to support methods like push, insert, and remove on models directly in .slint code, as well as for slint::Model

(Maybe using += to push.)

Original Description

Slint Vector type or modifyable Model within Slint**

Model has no way of appending or popping from within Slint. It would be good to have a Vector type that can do this.