Add feature to 'Link vertices' of paths

[humdinger]

It's quite common e.g. when you create a box like icon, that you later-on want to move a corner of that box. That's always a drag, because the corner is where at least three vertices from three paths come together: from the top, front and side pane.

Idea:
Let the user select vertices from two (or more) paths and offer in the context menu "Link vertices". From now on, if you move a linked vertice, its counterpart(s) are modified in the same way.

Of course there needs to be an "Unlink vertices" if you open the context menu of a linked vertice.

Different groups of linked vertices need to be indicated by another colour.

Clicking a linked vertice could show all paths with the counterparts in that group of linked vertices. To not complicate things too much, a vertice can only be part of one group, I suppose...

Currently, you can only select a single path from the list of Paths and only this one path is displayed on the canvas. This needs to change, so you can select multiple paths. When those selected paths are displayed, you can select the vertices to be linked.