[backport] Workaround for NPE in debugger variable view when using Eclipse Juno

Unfortunately, the way we keep track of the currently selected thread in the
Scala debugger isn't robust enough. Currently, we rely on a _selectionChanged_
event to be sent when the Debug Perspective is opened. This worked fine with
Eclipse Indigo, but it's no longer working with Eclipse Juno.

The workaround is to look for the first suspended thread if
``ScalaDebugger.currentThread`` is null. This is far from being a proper fix,
but it doesn't look like we have many alternatives. The Java debugger
implementation seem to be using a DebugContextProvider/Manager/Event/Listener
to know the thread associated to each variable. But using the debug context
classes would require some import work, so it would be preferable to delay this
for after the 3.0.0 final release.

The workaround was suggested by @skyluc, and he believes the only issue this
could lead to is visibility issues due to invoking a method call on the wrong
thread (in case more than one is suspended), and hence the executing thread may
see some stale values. Not an issue to be taken lightly.

Fix #1001585

backport to _release/3.0.x_
(cherry picked from commit 2de89febce7f9fb36820838b83f9ebd05fb0409d)