Improve "Undecodable sequence" warnings

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

lib/tools.cpp

@@ -5,6 +5,8 @@
 #include <QtDebug>
 
 
+Q_LOGGING_CATEGORY(qtermwidgetLogger, "qtermwidget", QtWarningMsg)
+
 /*! Helper function to get possible location of layout files.
 By default the KB_LAYOUT_DIR is used (linux/BSD/macports).
 But in some cases (apple bundle) there can be more locations).