Attribute grouping and locking

[sprocketc]

We currently display a row for each attribute of the selected elements on the properties sidebar.
The order of the attributes is specified under repath.studio.attrs.views with the attrs-order vector.

We need to find a way to introduce the idea of grouping.
Related attributes will be grouped and each category will have a locked? flag.
That is going to be useful for specific length attributes like width and height that need to be proportionally locked in some cases.

Grouped attributes should be visually distinguished from the rest.
A lock toggle button should be displayed next to them when the locked? key is present.
The required lock/unlock icons are already available.

This issue is a nice opportunity to investigate the way the custom attributes would work.
For instance, we could use a dedicated group with an add button at the bottom of the attributes.