Add API to get price scale's width

[timocov]

In #451 we added a way to convert price to coordinate (and vice versa), but it might be tricky, because the coordinate is local on time scale and it don't carry about price axis widgets what might adjust time scale.

We need to add a way to get price axis width so user can adjust coordinate from time scale by, for instance, width of left price axis and get exactly coordinate in chart widget.