Environment variable to force decorations from this plugin instead of using one from Qt

[MeguMario]

Newer version of Qt6 includes an older version of this plugin which do not have the new libadwaita colors. This PR added two more environment variables to avoid conflict with one from qt itself

With QT_WAYLAND_DECORATION=adwaita, both Qt and this plugin use the same key adwaita with Qt has higher priority.

With QT_WAYLAND_DECORATION=adwaita_new