Providing option to restrict element-picking to a particular type

This came up when I was preparing the coding assignments for CMU's DDG course.

Currently, the element picker picks up on elements of all sorts, including vertices, edges, faces, halfedges. On the other hand, in some assignments, students only need to select vertices on a mesh. Vertex selection is still doable, but on larger meshes it can require zooming in to more precisely select vertices.

It seems we've discussed this problem before (just logging it here.) One thing I'll add is that it might be helpful if the user could restrict to a certain element type by holding down a key while clicking.

没有指定实现文件、测试或入口点。首先定位 element-picker 代码，并查看 issue 中引用的早期讨论；当用户可以将 picking 限制为选定的元素类型（包括提议的按住按键交互），且不会失去现有的 picking 行为时，即可视为完成。