atcore: Add availableFirmwarePluginsChanged

Summary:
Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

ifirmware: Add CONSTATNT argument in Q_PROPERTY

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Reviewers: patrickelectric, rizzitello

Reviewed By: patrickelectric, rizzitello

Differential Revision: https://phabricator.kde.org/D18897