This commit adds support for linked axes between plots. One can specify
such linked limits using the new `linked_x_axis()` function (and the
variants for the Y axes) by specifying an `Rc<RefCell<ImPlotRange>>`
value, and passing clones of the same `Rc` to other plots.
The values within those `Rc` need to be kept persistent between frames,
hence the way to use this functionality is to keep a clone of the `Rc`
outside the frame-drawing function as part of the application state.
The regular limit setting API is unaffected.
