Select second handle with right click

[sayofan]

Hi,

First of all, thanks for this widget. I plan to use it to select a range of bytes from a file.
I ran into an issue were with two sliders and cross_each_other=False, I could not select and move the right handle.
If both handles are moved to the zero/left-most position, that means that the slider handles can not be moved anymore.

I introduced functionality to select the right-most handle with right click.

If enable_right_click_movement=True is given in init, right-click movement is set on.
Only implemented it for RangeSliderH, not RangeSliderV.

This may not be the best resolution for the issue I mentioned, but required less changes than other stuff I could think about.
Also, I left some comments in the code you may want to strip.
Other possibilities would be:

• When clicking on the track next to a handlebar, move handle in direction of the click on track;
• if bars have equal position on mousedown, select the handle based on the first movement
• if bars are both at 0, select second/right handle

Cheers!