Refactor how in progress drags are handeled.
Refactor how in progress drags are handeled.

In one of our apps we were consistently getting two `dragenter` events but only one `dragleave` event. This would cause the dropzone to be stuck in the active state. I coun't identify the cuase of the double events, I imagine it is somehow related to layout since another app we have seems to be working fine with seemingly identical components, just differences in how they are laid out / styled.

What this commit does is instead of tracking the number of times a `dragenter` event occurs (once or twice), it instead sets a `_dragInProgress` flag on the first `dragenter` event. Any further `dragenter` events are discarded until a `dragleave` event cancels the drag.

This fixes our app and seems to work in all the browsers I tested. (chrome and firefox).

paddle8/ember-plupload,tim-evans/ember-file-upload,tim-evans/ember-plupload,paddle8/ember-plupload,knownasilya/ember-plupload,gustaff-weldon/ember-plupload,tim-evans/ember-plupload,knownasilya/ember-plupload,gustaff-weldon/ember-plupload,tim-evans/ember-file-upload