[ENH]: Add setters for _AxLine._xy1, ._xy2, ._slope

[anntzer]

Problem

Currently the control points / slope of the artist returned by axline() cannot be (publically) modified after being instantiated. It would be nice if the relevant properties (xy1, xy2, slope) had setters (following normal Artist design).

For simplicity it is probably enough if we don't let one set xy2 if slope is set and vice-versa (i.e. whether axline is specified by 2 points or by point-and-slope is locked in). Note that while I do have a use case for changing a previously set xy1/xy2, wanting to switch between the two different representations seems rarer to me(?)

This would likely also make _AxLine public.

Proposed solution

No response