[FEATURE] Add functionality to allow FactorRange.start/end to be writable.

[gmagno]

For context: this issue being raised after an SO question kindly answered by @bryevdv: here.

Being able to set FactorRange.start/end can be useful when plotting categorical data, follows an example.

A ridge plot that gets clipped on top with p.y_range.range_padding = 0.0:

The same plot is not clipped but space is wasted with p.y_range.range_padding = 0.2

Finally, by manually repositioning the image I get what I want:

Bokeh docs has a nice example of a ridge plot that can be used to depict the issue.

Thanks in advance.

[bryevdv]

Will have to give some some thought, the best solution might be to afford independently configurable padding for start and end, but leave start and end themselves read-only as they currently are.