Improve SQL Browser Example - Coding style

SQL Browser is an highlighted Application Example. Anyhow, its code base is not implementing best practices:

• Uses Qt module headers instead of individual class headers
• Code uses connect-by-name instead of explicit connect() (clazy-connect-by-name)
• browser.cpp: Emit signal inside constructor (clazy-incorrect-emit)
• Replace QObject::tr() antipattern with a more meaningful context
• main.cpp: Use QCommandLineParser instead of manual argc, argv handling