Make the SectionDelegate look just a little bit better

This has insets on it - probably from qqc2-desktop-style - and makes it
look extremely bad when scrolling through messages as the background
size doesn't match. Now the insets are set to zero, except for topInset.
This is done to work around a visual bug where you can see a one-pixel
line right above the SectionDelegate when scrolling.

(cherry picked from commit ab5585cd06)

src/timeline/SectionDelegate.qml

@@ -23,6 +23,11 @@ QQC2.ItemDelegate {
     topPadding: Kirigami.Units.largeSpacing
     bottomPadding: 0 // Note not 0 by default
 
+    leftInset: 0
+    rightInset: 0
+    topInset: -1 // This -1 is intentional to stretch the background one more pixel to prevent a visual bug when scrolling
+    bottomInset: 0
+
     contentItem: ColumnLayout {
         spacing: Kirigami.Units.smallSpacing
         Layout.fillWidth: true