applets/systemtray: fix xembed icon not receiving click events when using Qt scaling
On X11, when using Qt scaling, mapToGlobal will return a scaled position which is unusable in `SNIProxy::sendClick`. This fixes the bug by multiplying the position by devicePixelRatio. BUG: 456168 FIXED-IN: 5.27 (cherry picked from commit 8b1250ac)