Feed the Properties panel with editable layer-specific parameters

The Properties panel will let us adjust the location and properties of shapes like width, height, radius, number of n-gon points, etc. We'll need to use #147 to send from Rust the JSON layout of input widgets, then respond with changes to input widgets and update the Document accordingly. ~~Depends on #137 and #147.~~

Complexity: 4
Involves: Frontend (JS), Editor (Rust), Document (Rust)