Ensure most of our dialogs have cancel buttons on the right

This is standard for KDE dialogs, but we weren't super consistent about
it. Most of this is porting to DialogButtonBox, and using buttonRole. I
also made sure the close button is hidden in cases where we have a
cancel button already.

I also took the time to clean out some cruft like manually calling
close() and such. I probably missed a lot more spots, but this is a good
improvement.

src/app/qml/ConfirmLeaveDialog.qml

@@ -16,19 +16,18 @@ Kirigami.PromptDialog {
     title: root.room.isSpace ? i18nc("@title:dialog", "Confirm Leaving Space") : i18nc("@title:dialog", "Confirm Leaving Room")
     subtitle: root.room ? i18nc("Do you really want to leave <room name>?", "Do you really want to leave %1?", root.room.displayNameForHtml) : ""
     dialogType: Kirigami.PromptDialog.Warning
+    standardButtons: QQC2.Dialog.Cancel
 
-    onRejected: {
-        root.close();
-    }
+    onAccepted: root.room.forget()
 
     footer: QQC2.DialogButtonBox {
-        standardButtons: QQC2.Dialog.Cancel
-
         QQC2.Button {
-            text: i18nc("@action:button", "Leave Room")
-            QQC2.DialogButtonBox.buttonRole: QQC2.DialogButtonBox.AcceptRole
+            text: i18nc("@action:button Leave this room/space", "Leave")
             icon.name: "arrow-left-symbolic"
-            onClicked: root.room.forget();
+
+            onClicked: root.accept()
+
+            QQC2.DialogButtonBox.buttonRole: QQC2.DialogButtonBox.AcceptRole
         }
     }
 }