Share RemoteKeyboard implementation

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