applets/systemtray: open context menu on pressed for SNI
With a workaround for QTBUG-59044, the same context menu will no longer be opened twice.
The description in the original fix by @kmaterka is not true since XEmbedSNIProxy already sends a mouse release event after a mouse press event.
BUG: 409768 FIXED-IN: 5.27
Edited by Fushan Wen