Allow drag start condition customization

[csisy-bt4w]

We want to only allow dragging by the left mouse button and not the other ones.

Currently, we have no way to stop a draggable element to enter the dragging state.

zrender/src/mixin/Draggable.ts

Line 45 in 4bcc023

_dragStart(e: ElementEvent) {

Possible solutions:

• Emit the event first, then check if the event was cancelled by the handler or not.
• Provide a separate callback that receives the event and returns a boolean
• Allow the draggable property to be a function as well which receives the event and returns the currently known results (boolean | 'horizontal' | 'vertical')