MobX State Tree Implementation for Circle Drawer

It would be great to have an alterntaive implementation to [the traditional one](https://github.com/eugenkiss/7guis-React-TypeScript-MobX/blob/master/src/app/guis/circles/drawer-traditional.tsx) using [MobX State Tree](https://github.com/mobxjs/mobx-state-tree). To that end, I already structured the code such that the presentational layer doesn't have to be reimplemented. Here's the [placeholder file](https://github.com/eugenkiss/7guis-React-TypeScript-MobX/blob/master/src/app/guis/circles/drawer-mst.tsx).

Start by comparing src/app/guis/circles/drawer-traditional.tsx with the placeholder src/app/guis/circles/drawer-mst.tsx, then review the linked MobX State Tree documentation. Implement the Circle Drawer state layer in the placeholder while reusing the existing presentational layer. Done means the drawer has a working MobX State Tree implementation without reimplementing its presentation.

Written by the indexing model from the issue text.