xdgdesktopfile: Allow non-detach starting
xdgdesktopfile: Allow non-detach starting

Allow runtime overriding of detachment of processes by setting the
QTXDG_START_DETACH_TRULY. Default value is true [1].
With this the application (using libqt5xdg) can correctly send
termination signal to child processes on the application exit.

libqtxdg/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg,lxde/libqtxdg