[SMS App] Allow scrolling up to load and display older messages

Summary:
Scroll up to show older messages

Newly received messages will not force the view to the bottom unless the new message is being added "very close" to the visible area

Test Plan:
Message History:

• Open conversation
• Scroll/Drag up to load older messages

New Message:

• Open conversation
• Scroll to bottom
• Verify that a newly-received or newly-sent message is added to the GUI
• Scroll up
• Verify that sending/receiving a message does not disturb the view
• Scroll back to verify that the new message was indeed added to the list

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: apol, nicolasfella, kdeconnect

Tags: KDE Connect

Maniphest Tasks: T9556

Differential Revision: https://phabricator.kde.org/D15979