Fixes XdgDesktop::isShow() caching mechanism
Fixes XdgDesktop::isShow() caching mechanism

The cache mechanism was totally broken. It stored the last mIsShow value.
And that's it. It didn't take into account the environment.
It didn't work with straightforward code like this:
for (int i = 0; i < N; ++i)
{
    if (file->isShown(envs.at(i)))
    {
        show = true;
        break;
    }
}

Closes lxde/lxqt#563

libqtxdg/libqtxdg,libqtxdg/libqtxdg,DrTobe/libqtxdg,luis-pereira/libqtxdg,lxde/libqtxdg,lxde/libqtxdg,luis-pereira/libqtxdg,DrTobe/libqtxdg,DrTobe/libqtxdg