With this plugin, all Qt-based programs can adopt settings of
LXQt, such as the icon theme.
To use the plugin in Qt5, we have to export the environment
variable QT_QPA_PLATFORMTHEME=lxqt. Then every Qt5 program
can load the theme plugin.
If, for some unknown reasons, the plugin is not loaded, we can
debug the plugin by exporting QT_DEBUG_PLUGINS=1.
Then, Qt5 will print detailed information and error messages
about all plugins in the console when running any Qt5 programs.