Qt: Add extcap configuration to Welcome screen context menu

For extcap interfaces, add launching their configuration dialog to the right-click context menu in the list of interfaces on the welcome screen. That is a larger amount of real estate to possibly click, and might be more easily discoverable to some users than clicking on the extcap icon.

Related to #13287, #19817 (we need something like this to configure extcaps if we want to have the option of a QListView in IconMode for the interfaces.)