Add options bar widgets for tools to control tool behavior

[TrueDoctor]

We need to implement a system for manipulating tool options such as stroke width and fill.
We will eventually replace this with a more sophisticated system supporting plugins as well.

The idea is to use Vue to render the input widgets in the viewport options bar and (at least for now) just generate update messages for tool options akin to the way colors are currently updated.

• The tool-specific options bar content (basically a container <div v-if="...">...</div>)
• A unified approach to routing options changes to the backend, and the state of those options back to the frontend
• A proof-of-concept implementation for setting the number of sides on the N-gon of the Shape Tool

Blocked on #190.

Complexity 3
JS, Rust