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

applets/systemtray/package/contents/ui/PlasmoidPopupsContainer.qml

@@ -42,7 +42,8 @@ StackView {
             activeApplet.fullRepresentationItem.anchors.centerIn = undefined;
             activeApplet.fullRepresentationItem.anchors.fill = undefined;
 
-            if (activeApplet.fullRepresentationItem instanceof PlasmaComponents3.Page) {
+            if (activeApplet.fullRepresentationItem instanceof PlasmaComponents3.Page ||
+                activeApplet.fullRepresentationItem instanceof PlasmaExtras.Representation) {
                 if (activeApplet.fullRepresentationItem.header && activeApplet.fullRepresentationItem.header instanceof PlasmaExtras.PlasmoidHeading) {
                     mainStack.appletHasHeading = true
                     activeApplet.fullRepresentationItem.header.background.visible = false