Fixed Drag'n'Drop for Firefox 54

[SpazzMarticus]

Brief description of the changes

Fix for #1862
Reordered checks in leavingDocumentOut in order to try use e.x and e.y, before falling back on e.relatedTarget.

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

Windows 8.1 x64 - Worked with Firefox 47 and 54.
I don't know if this has anything to do with it: https://bugzilla.mozilla.org/show_bug.cgi?id=962251)
(Works in Chrome 59 too.)

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

Did not find any unit tests for drag and drop.

[cubiclesoft]

A change in drag-and-drop support - one of the core features of a file uploader - needs to be tested widely across all browsers with as many versions as is reasonably possible. This PR seems like a rather significant change to just accept into mainline with such minimal testing. A less invasive approach would be to isolate just Firefox using the existing qq.firefox() check.

[maximilianschmid]

Tested patch successfully with Fine Uploader 5.15.0 on

• IE11 Win732bit
• FF 55 Mac
• FF 55 32bit Win732bit
• FF 30 Win732bit
• Safari 10.1
• Google Chrome 60 Mac & Win

[rnicholus]

I’d like some explanation regarding the details of this fix. It’s not clear to me why this fixes the issue, which I have not verified myself.

[SpazzMarticus]

My fix does not work correctly any more in Firefox 56 (x64, Win) any more, because when leaving the browser window I get this error: Permission denied to access property "x"
It solves the anoying flickering problem, but does not hide the Drop-Zone when leaving the browser window.

Also in Firefox 56 (x64, Ubuntu 17) the x and y are not set to 0 or negative values when leaving the browser window.

I hope you can fix this problem!