Add a “set start point” command

[eliheuer]

This is an updated version of a previous PR I closed here: #260

This attempt is based on some feedback from the Zulip given by @cmyr:

The identity of the 'start point' is determined by a convention: In an open path, it is the first point in the list, and in a closed path (which is the only case where changing the start point really makes sense) it is the last point in a list.
So to implement this:
ensure that the path is closed
ensure that there is one selected point, and it is an on-curve
rotate the points to put this point at the end of the list
I would start by adding a method in point_list.rs, PathPoints::make_start(&mut self, point: EntityId). Then you'll want to call this from edit_session (likely via Path), passing in the id of the current selection.

Currently the command is in the Paths menu and works with the Shift+Ctrl+P command. If multiple points are selected it logs that info and does nothing, if one point is selected it calls make_start from Path.

I feel like I can figure out most of what is needed with a bit more work, but I'm a bit lost on the best way to do this. Specifically how to get the right EntityId in editor.rs and how to rotate the points.

Any feedback is welcome, I think I can figure most of this out on my own, it just takes me a long time.

Thanks!