Qt: Add a WiresharkDialog convenience class.

Add WiresharkDialog, a common base class for dialogs centered around
capture files. Make it a parent of Capture File Properties, Traffic
Table, Conversations, and Endpoints.

Rename CaptureFile::read_only_ to file_closed_. Add methods to
WiresharkApplication for generating consistent window titles.

Change-Id: Idc771556d8192e60f85dddc08fc4757698dee257
Reviewed-on: https://code.wireshark.org/review/6097
Reviewed-by: Gerald Combs <gerald@wireshark.org>