Rework MessageDelegate in cpp

Rework MessageDelegate to be mostly a cpp class. This allows us to only load the components that are actually needed saving memory.

In testing using memtest it saved ~30% versus the current implementation.

memorytests/Main.qml

@@ -23,6 +23,7 @@ QQC2.ApplicationWindow {
         height: root.height
 
         contentItem: ListView {
+            cacheBuffer: 1000000
             model: messageFilterModel
 
             delegate: EventDelegate {