Fix backdrop for center position

[ktriek]

Fixed backdrop for positions:
Center_center
Left_center
Right_center

[artemsky]

It does not fix that issue.
The reason is that now all toast will append at the bottom. Right from the center of screen

How it should be?
Toasts should be re-centered every creation. So all of them could fit at the screen center

How to?
I think we should calculate snotify-component height, and append negative margin after each ngAfterViewChecked

p.s.: i am really knew about this issue. It css bug for now. They should fix it next versions.

But for i appreciate your pull request, and i'll take it right after i find a way to append negative margin in a right way.

[ktriek]

@artemsky thank you very much for the review and the great work you are doing.