Improve support for user attributes

We can add user-defined attributes to meshes, but working with them is decidedly an afterthought.

Instead, user attributes should be more like custom properties. In addition to type, we should be able to specify a subtype (e.g. distance, angle), range, and display name, and they should show up in the "Item" panel when a relevant item is selected. (Float attributes, at minimum. And editing an attribute on multiple elements at once should work like it does for "built-in" attributes.)

Possibly these properties should be defined at the file level so their metadata can easily be shared across many objects.