Fix for problem with 'Enqueue for Download' and

- Problem with 'Enqueue for Download' has almost been fixed; only
problem left is some null values in the error console when the
queueingStatusManager is cancelled.
- install.rdf: minVersion changed to 12.0 and maxVersion to 20.0
-globals.js: suppressWarningMessages added; mention of "BTYubeD" in
error report request messages.
- selectionManager.dtd, selectionManager.xul and selectionManager.js:
added an option to suppress warnings like those shown during 'Enqueue
for Download'.