Extend Tracker scripting capabilities

* Allow EXECUTE on the Folder property to open a window for the passed
ref and return a BMEssenger targetting that window.
* Allog GET on the Folder property to return a messenger to the matching
window, if one is already open.
* Make scripting support mandatory and remove the define allowing to
disable it.
Rewrite "open containing folder" in a safer way.

* Tracker has support for this in its RefsReceived handler, which
results in simpler code than going through BRoster to open the folder.
* Avoids a race condition, possible confusion of Tracker windows with
the same title, and makes the code more readable (and working).
* Fixes #11008.

Thanks to Axeld for pointing the support in RefsReceived, which wasn't
used anywhere outside of Tracker, yet.