platforms/xcb: Handle Xwayland restarts better

Vlad Zahorodniirequested to merge
work/zzag/atoms-cache into master

If Xwayland is restarted, the same xcb_connection_t pointer can be used by both the old and new connections.

If that happens, the old atoms will be used too, however, that will be wrong because with the new connection, atom values may be different.

The long term goal is to move the relevant window manager bits to kwin itself.

This fixes X11 apps misbehaving after Xwayland crashes.

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