Summary: Extract RemoteKeyboard.qml into declarativeplugin to be able to use it from both plasmoid and app without code duplication

Reviewers: #kde_connect, hkaelberer, apol

Reviewed By: #kde_connect, apol

Subscribers: kdeconnect, mtijink, apol, #kde_connect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D12670