Convert TimelineDelegate to cpp

There are 2 main reason for doing this:
1. Because I can, I wanted to see if I could do it
2. It gets rid of the janky qml re parenting stuff so should be faster.

src/timeline/ReadMarkerDelegate.qml

@@ -8,6 +8,8 @@ import QtQuick.Layouts
 import Qt.labs.qmlmodels
 import org.kde.kirigami as Kirigami
 
+import org.kde.neochat
+
 TimelineDelegate {
     id: root
 
@@ -16,6 +18,11 @@ TimelineDelegate {
         temporaryHighlightTimer.start();
     }
 
+    width: parent?.width
+    rightPadding: Config.compactLayout && root.ListView.view.width >= Kirigami.Units.gridUnit * 20 ? Kirigami.Units.gridUnit * 2 + Kirigami.Units.largeSpacing : Kirigami.Units.largeSpacing
+
+    alwaysFillWidth: Config.compactLayout
+
     contentItem: QQC2.ItemDelegate {
         padding: Kirigami.Units.largeSpacing
         topInset: Kirigami.Units.largeSpacing