Fix a "recent items" crash.

Use a QMutableListIterator instead of a foreach loop so that we can
safely remove items from a list.

While we're here, make sure that RecentFileStatus threads use a
Qt::QueuedConnection when emitting signals across threads and try to
isolate the filename string.

Change-Id: I3fbb65a1727133f4557026decab5084a3faec2d4
Reviewed-on: https://code.wireshark.org/review/2966
Reviewed-by: Gerald Combs <gerald@wireshark.org>