TabPane: issues when/after dragging tab header

For a runnable example, see JDK-8252236

Basically, there are different issues (not sure if/how they are related) for dragging off the leading/trailing edge

A: dragging a header off the trailing edge

1. has no visual clue as to where it will be inserted
- scroll the tabs such that there are headers off the trailing edge
- drag a header to and over the trailing edge
- expected: a visual clue as to where it will be inserted, f.i. by scrolling the header before the current drop location into view
- actual: only one (or two) headers are scrolled into view

2. dragged header not visible after drop
- drop the header
- expected: dropped header is visible
- actual: dropped header is not visible

B: it is not possible to drag a header off the leading edge, even if there are still tabs before

- select header near the middle/end (to ensure there are invisible headers off the leading edge)
- drag a visible header to the left and beyond the leading edge
- expected: possible to drag header off the leading edge and dragged-over headers are scrolled into view
- actual: drag stops at leading boundary, limiting the insertion index to the current first

A2 is also noted in Kevin's comment to JDK-8187074 (which might have been fixed at that time and regressed? Didn't dig, though). Anyway, this is partly fixed by JDK-8252236, by ensuring the visibility of the dropped header after the drag, but doing nothing to the scrolling during the drag. And not certain whether the drop happens at the correct location.