Use the elements z-index as base for z-index calculations when resizing and dragging

[ClemensSchneider]

When resizing and dragging, a fixed z-index was used and set (in case of resizing) on the event-element. If the z-index was altered by the developer, it was reset after resizing.
With this patch, the original z-index of the event-element is taken into account.

[arshaw]

I made an alternate solution in 664ec8f, and committed to the upcoming release branch.

In your patch, to handle customized z-indexes, you are setting the dragged z-index to be the original +1. But what we really want to do is to make it appear above all other events, so we need to set the z-index to higher than all the other events' z-indexes, which could be custom as well.

That solution could get hairy, so my solution was to take the inline z-index styles on the event elements, and put them into the stylesheet, so they could be more easily overridden.

[arshaw]

just released in v1.6.3