Add a gui to select jump targets

Currently, we just take the current chapter name as a jump target. This means the headings have to have the exact name, or we have to add <div name="...">...</div> tags to our cells.

We should add a chapter select dropdown to the editor:

One way would be to add a column for the jump target (as in the mockup above). Another option is to add it below the chapter in a separate block. This way we could add multiple "actions" to a chapter beyond jumping to a cell, such as revealing cells or changing variables.

(Currently, I prefer option 1, since we don't have other actions than "jump", and it is clearer.)