Fix weird scrolling behavior in room list

You may have noticed that the room list acts a bit... odd. You usually
can't scroll all the way down with a scrollwheel, it just
stops. It's possible to continue a little bit more with the scrollbar.
And sometimes the scrollbar doesn't know how big it's actually supposed
to be, commonly occuring when switching from a large room list to a
smaller one.

I narrowed it down the same usual problem with views in QtQuick:
variable sized delegates! Using GammaRay I figured out that for the
delegates currently in use they're slightly different: 46 pixels for
regular room delegates, 42 pixels for section headers and the "Find your
Friends" button was also different.

*Technically* TableView (and by extension TreeView) is supposed to allow
variable-sized delegates, and we should be able to advertise that with
rowHeightProvider. I tried a bunch of different solutions and none of
them worked reliably, so I took the usual sledgehammer approach of
making all of the delegates the same size.

This fixes all of the obvious bugs with the room list I could see, with
the one visual downside of making the section headers slightly taller.
But since I spent some time improving the tap targets, this is only a
visual change and not a functional one.

I also made sure to test it in compact mode, and everything shrinks as
expected.

src/rooms/RoomTreeSection.qml

@@ -6,8 +6,11 @@ import QtQuick.Controls as QQC2
 import QtQuick.Layouts
 
 import org.kde.kirigami as Kirigami
+import org.kde.kirigamiaddons.delegates as Delegates
 
-QQC2.ItemDelegate {
+import org.kde.neochat
+
+Delegates.RoundedItemDelegate {
     id: root
 
     required property TreeView treeView
@@ -36,29 +39,31 @@ QQC2.ItemDelegate {
     Keys.onSpacePressed: root.treeView.toggleExpanded(row)
 
     contentItem: Item {
-        implicitWidth: layout.implicitWidth
-        implicitHeight: layout.implicitHeight
+        implicitHeight: Math.max(layout.implicitHeight, Kirigami.Units.gridUnit + (NeoChatConfig.compactRoomList ? 0 : Kirigami.Units.largeSpacing * 2))
 
         RowLayout {
             id: layout
 
-            width: root.contentItem.width
+            anchors.fill: parent
 
             spacing: 0
 
             Kirigami.ListSectionHeader {
-                Layout.fillWidth: true
                 visible: !root.collapsed
                 horizontalPadding: 0
                 topPadding: 0
                 bottomPadding: 0
                 text: root.collapsed ? "" : root.displayName
 
-                onClicked: root.treeView.toggleExpanded(row)
+                onClicked: root.treeView.toggleExpanded(root.row)
+
+                Layout.fillWidth: true
+                Layout.alignment: Qt.AlignVCenter
             }
             QQC2.ToolButton {
                 id: collapseButton
-                Layout.alignment: Qt.AlignHCenter
+
+                Layout.alignment: Qt.AlignCenter
 
                 icon {
                     name: root.expanded ? "go-up" : "go-down"