stopPropagation breaks blur() events elsewhere #939

Steps to reproduce:
Given: I have an input (unrelated to rc-slider), and a <Slider> element

1. Click into the input. Expected: cursor blinks in input, focus is on input
2. Click the slider drag handle
   Expected: a blur() event is called on the input
   Actual: no blur() event is called

I suspect this is because stopPropagation is called here. What is the reason for this call?

I will gladly open a PR to resolve this problem, but before I do it would be best for me to understand the problem that stopProgation is solving.

Thanks!