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/RoomListPage.qml

@@ -139,6 +139,7 @@ Kirigami.Page {
             contentItem: TreeView {
                 id: treeView
                 topMargin: Math.round(Kirigami.Units.smallSpacing / 2)
+                bottomMargin: Math.round(Kirigami.Units.smallSpacing / 2)
 
                 clip: true
                 reuseItems: false
@@ -147,6 +148,17 @@ Kirigami.Page {
 
                 selectionModel: ItemSelectionModel {}
 
+                // This is somewhat of a hack.
+                // If we don't calculate this for TableView, then the content area doesn't quite scroll down far enough to cover the margins.
+                rowHeightProvider: row => {
+                    // NOTE: This padding value should be kept in sync with the padding value of our delegates.
+                    const padding = Kirigami.Units.mediumSpacing;
+                    // NOTE: This calculation should be kept in sync with the height value of our delegates.
+                    return Kirigami.Units.gridUnit + (NeoChatConfig.compactRoomList ? 0 : Kirigami.Units.largeSpacing * 2) + padding * 2;
+                }
+
+                // NOTE: For any future delegate spelunkers, please *keep the delegate heights in sync*.
+                // If you fail to do so, weird scrolling behavior begins to manifest.
                 delegate: DelegateChooser {
                     role: "delegateType"
 
@@ -181,8 +193,8 @@ Kirigami.Page {
                         delegate: Delegates.RoundedItemDelegate {
                             text: i18nc("@action:button", "Find your friends")
                             icon.name: "list-add-user"
-                            icon.width: Kirigami.Units.gridUnit * 2
-                            icon.height: Kirigami.Units.gridUnit * 2
+                            icon.width: Kirigami.Units.gridUnit + (NeoChatConfig.compactRoomList ? 0 : Kirigami.Units.largeSpacing * 2)
+                            icon.height: Kirigami.Units.gridUnit + (NeoChatConfig.compactRoomList ? 0 : Kirigami.Units.largeSpacing * 2)
 
                             onClicked: (Kirigami.PageStack.pageStack as Kirigami.PageRow).pushDialogLayer(Qt.createComponent('org.kde.neochat', 'UserSearchPage'), {
                                 connection: root.connection