feat(dnd.js): add dragEnter and dragLeave callbacks

[HerringtonDarkholme]

Brief description of the changes

Currently fine-uploader dnd only toggles class name on targetElement. However, users need more advanced UI control when dragging/drop.

What browsers and operating systems have you tested these changes on?

Firefox

Have you written unit tests? If not, explain why.

No. I didn't find other callbacks for dnd like processingDroppedFilesComplete in unit test. So I think it might be better to ask maintainers how to add it.

[singhjusraj]

@rnicholus this seems like something we can easily merge. Seems handy to have those two callbacks

[rnicholus]

@SinghSukhdeep I no longer have any time to dedicate to this project, at all. You've been pretty attentive to the project, and perhaps you'd be interested in taking over. If so, let me know and I'll make sure you have all of the appropriate permissions. I'd still be available if you have questions, but turnaround time might not be ideal.

[singhjusraj]

I hear you, I will look at this closely soon and will try my best

[HerringtonDarkholme]

Thank you for taking up the project!

[singhjusraj]

Hoping to get some time soon to look at all the pending PRs

[singhjusraj]

Everything looks good.
This also requires docs update for drag and drop page
Can you please add that as well and I'll merge this

[singhjusraj]

Requires docs updates as well

[HerringtonDarkholme]

Thanks!