Don't show flash of pairing buttons when opening a device

Summary:
The buttons are now hidden, and only showed once we know that the device
is unpaired.

Test Plan: Pairing, pairing request and opening a device still work.

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Subscribers: KDE Connect

Differential Revision: https://phabricator.kde.org/D11627