Call signals instead of signal handlers

Directly calling signals is the supported way to send signals.
Calling signal handlers worked in the past, but will be phased out in
the future (https://bugreports.qt.io/browse/QTBUG-120573).

smsapp/qml/ConversationList.qml

@@ -163,7 +163,7 @@ Kirigami.ScrollablePage
 
             Shortcut {
                 sequence: "Ctrl+N"
-                onActivated: newButton.onClicked()
+                onActivated: newButton.clicked()
             }
         }
     }

smsapp/qml/SendingArea.qml

@@ -88,7 +88,7 @@ ColumnLayout {
                             if (event.modifiers & Qt.ShiftModifier) {
                                 messageField.append("")
                             } else {
-                                sendButton.onClicked()
+                                sendButton.clicked()
                                 event.accepted = true
                             }
                         }