applets/systemtray: fix glitch for some applets with headers
PlasmoidPopupsContainer.qml has special code to handle merging headers from applets if those applets use a PlasmaComponents3.Page instance as their FullRepresentation. However this breaks for any applets that use PlasmaExtras.Representation, which is a subclass of it that we have been porting applets to over time, because it offers more features in a single location. This caused the detection to break, so such applets woudl have their headers improperly merged. Explicitly detecting the use of PlasmaExtras.Representation as well fixes it. BUG: 438178 FIXED-IN: 5.24