Improve "No devices" warning

Summary:
Move the "No devices available" text in smsapp from combobox to the ConversationList.

P.S.
It's a GCI task.

Test Plan:

• Build and run smsapp
• Try pairing and unpairing the device
• See the difference

Reviewers: KDE Connect, sredman

Reviewed By: KDE Connect, sredman

Subscribers: sredman, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D16467