workaround qt ref counting bug seen triggered often in libtaskmanager

not patching libplasma as i'd like to keep this mess limited to taskmanager related code
rather than burdoning all DataEngines with it. this requires today's KDE/4.7 libplasma to
work as expected: sourceRemoved was not being emitted for direct-deleted containers.

BUG:286353
REVIEW:103247