[SMS App] Make SMS message field scrollable

## Summary

This patch makes SMS message field scrollable and limits its height to ⅓
of window height (or ⅔ for smaller windows) so messages in conversation
are shown even when writing very long messages.

## Test Plan (and screenshots)

<small>These screenshots are in APNG and WEBP formats. Download them if
your browser does not support this format.</small>

### Before:

Message field height was not limited. Message field could be even higher
than window. That makes first lines of this field and messages in
conversation inaccessible.

![grab-noscroll.apng](/uploads/70a44ec3abd0e0707cf69f53ee832542/grab-noscroll.apng)

### After:

Message field height is limited to ⅓ or ⅔ of window height. When text
content reaches this threshold, scroll bars will show up.

![grab-scroll.webp](/uploads/489b9bb197a092f715bc2160acc7fd42/grab-scroll.webp)

smsapp/qml/ConversationDisplay.qml

@@ -199,15 +199,22 @@ Kirigami.ScrollablePage
         RowLayout {
             anchors.fill: parent
 
-            TextArea {
-                id: messageField
+            ScrollView {
                 Layout.fillWidth: true
+                Layout.maximumHeight: page.height > 300 ? page.height / 3 : 2 * page.height / 3
+                contentWidth: page.width - sendButtonArea.width
+                clip: true
+                ScrollBar.horizontal.policy: ScrollBar.AlwaysOff
+
+                TextArea {
+                    anchors.fill: parent
+                    id: messageField
                     placeholderText: page.isMultitarget ? i18nd("kdeconnect-sms", "Replying to multitarget messages is not supported") : i18nd("kdeconnect-sms", "Compose message")
                     wrapMode: TextArea.Wrap
                     topPadding: Kirigami.Units.gridUnit * 0.5
                     bottomPadding: topPadding
                     selectByMouse: true
-                background: Item {}
+                    topInset: height * 2 // This removes background (frame) of the TextArea. Setting `background: Item {}` would cause segfault.
                     Keys.onReturnPressed: {
                         if (event.key === Qt.Key_Return) {
                             if (event.modifiers & Qt.ShiftModifier) {
@@ -219,8 +226,11 @@ Kirigami.ScrollablePage
                         }
                     }
                 }
+            }
             
             ColumnLayout {
+                id: sendButtonArea
+
                 ToolButton {
                     id: sendButton
                     Layout.preferredWidth: Kirigami.Units.gridUnit * 2