Qt: Add methods to CaptureFile and WiresharkApplication.

Add isValid, fileTitle, and retapPackets methods to CaptureFile. Add
application name and title separator convenience methods to
WiresharkApplication.

Convert traffic tables, conversations, and endpoints to use CaptureFile
directly and to let the user know when the capture file is closed. Set
the file dialog titles while we're here.

Change-Id: I990392786d3833e1e0b3638aa2d34a5ada39fa13
Reviewed-on: https://code.wireshark.org/review/5957
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>