* Allow cursors to be linked
* Link cursors in demo
* Refactor cursor memory to deal with removal of plots
* Refactor PlotItem::on_hover to produce a list of cursors
* Use a separate `LinkedCursorsGroup` type to link cursors.
* Refactor `Cursor`.
* Inline `push_argument_ruler` and `push_value_ruler`.
* Update documentation
* Update doc reference