Display an overlay on drop area. Implements #10369

Basically displays a div with a border around.
Needed to change drop event a little bit since there is
an issue with the sidebar if only `$(document)` is used.


Do not merge yet! On mobile, the overlay does not cover all list.

@owncloud/designers 
I'd prefer if you used an absolutely positionned "drophere" element instead of wrapping the whole app inside, which is likely to cause side-effects.

Just have  `<div id="drophere"></div>" as an independent element inside the "viewcontainer".
Then use CSS classes to show/hide it.