Uses new debug view selection support

In Eclipse 4.x, a new way to follow the current selection in the debug view was implemented,
as debug specific support.
The old way was slightly broken, and was no more working for selection changes not triggered
by the user. A temporary fix was implemented in Scala 3.0.x, to support Eclipse 3.x and 4.x.
This commit replace the old code and the fix by a solution using the new support.

Fixes #1001585
Need to be backported in 3.0.x-juno.
Merge pull request #511 from skyluc/issue/current-selected-thread-1001585

Uses new debug view selection support