Using Qt logging utilities instead of printf so that those warnings
can be enabled/disabled. Those warnings are disabled by default. To
enable them, use QT_LOGGING_RULES="qtermwidget.debug=true".
Also improve the formatting. An example warning:
Undecodable sequence: "\u001B[22;0;0t"
Closes https://github.com/lxqt/qterminal/issues/640