applets/systemtray: Do not open context menu on mouse pressed for SNI
The preferred is to show context menu on mouse pressed event, not click, but this causes few issues for SNI.
Some legacy applications still relies on XEmbed to show tray icon. We use xembedsniproxy to relay the mouse press signals to the XEmbed client but some applications (including GTK) validate the mouse state. Mouse released state is expected, but as it is still pressed context menu does not show.
BUG: 409768 FIXED-IN: 5.24.3