KateTabBar: enable changeCurrentOnDrag

Time to catch up to Okteta after 3 years ;)

And also Dolphin, which though uses some custom support, filtering out MIME types the view can support. Not sure if that is needed though, people might usually only try to drag things to Kate views which they assume can be dropped as text. And if it cannot they will find out seconds later still when hovering over the view switched to.