Qt: Convert more dialogs to WiresharkDialog.

Make the Sequence, IO Graph, Statistics Tree, and VoIP Calls dialogs
subclasses of WiresharkDialog.

Remove "Stats Tree" from Statistics Tree dialog titles. Don't complain
if the user opens more than one instance of the dialog.

Use the applicationName property in WiresharkApplication instead of a
separate variable.

Add a preexisting item to the IO Graph bug list (hovering when the file
is closed clears the graph).

Change-Id: I8411a25305d00b16e0d4a82fa50a9bad5c85b239
Reviewed-on: https://code.wireshark.org/review/6125
Reviewed-by: Gerald Combs <gerald@wireshark.org>