Emit contextualActionsAboutToShow before showing the applet's contextualActions menu.

This is done by ContainmentInterface::mousePressEvent, but not in this
code path, which is hit for applet panel popups.

Folder View relies on this to update actions ahead of the menu being
opened.

BUG:366294

src/scriptengines/qml/plasmoid/appletinterface.cpp

@@ -758,6 +758,7 @@ bool AppletInterface::eventFilter(QObject *watched, QEvent *event)
             }
 
             QMenu *desktopMenu = new QMenu;
+            emit applet->contextualActionsAboutToShow();
             ci->addAppletActions(desktopMenu, applet(), event);
 
             if (!desktopMenu->isEmpty()) {