waylandclipboard: Update QClipboard when gaining focus

A process that owns the clipboard via KSystemClipboard (wlr-data-control) and tries to read the clipboard via QClipboard will deadlock itself and eventually times out because the read is done blocking and the send event is only received afterwards by WaylandClipboard. When Qt knows it owns the keyboard it can get the data directly, so the idea is to set QClipboard if possible. However this can be done only when the Application has focus. Unfortunately inside Qt window system events are delieverd asnycronously while QClipboard uses signal to indicate changes, this makes QGuiApplication::focusWindowChanged not useable here because it happens to late - after the QClipboard signal. For this reason we bind to the wl_seat and track focus ourselves. QClipboard takes ownership of QMimeData that is passed into it and so does KSystemClipboard and ultimately DataControlSource which now uses unique_ptr to signify this and make it clear that ownership is transferred when the QClipboard is set. BUG:442521 BUG:454379