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.
- This commit also fixes an issue with legend rotation and orientation
always being set to values even if the user did not specify any.
This broke the interactive configuration of where the legend should
be.
