First draft for drag and drop

We can change feed category by drag and drop! Need improvements...

See https://github.com/marienfressinaud/FreshRSS/issues/646
Add dataTransfer property to event for JQuery

See https://github.com/marienfressinaud/FreshRSS/issues/646
Improve drag and drop

- Refactoring
- Better design
- Item doesn't disappear if action is not completed

See https://github.com/marienfressinaud/FreshRSS/issues/646
Hack for dragleave (triggered on children!)

See https://github.com/marienfressinaud/FreshRSS/issues/646
Fix box when no feed in category

See https://github.com/marienfressinaud/FreshRSS/issues/646
Merge branch '646-new-cat-system' into dev