[T4760] Add readonly feature for Worksheet: if user open worksheet file and don't have backend for it (or backend requirements are not satisfied), worksheet assigned as 'ReadOnly', that means, what entries are become non-editable and non-executable and also some buttons, like 'Restart Worksheet', 'Show Backend Help', etc. are hided.

Closes T4760
Differential Revision: https://phabricator.kde.org/D16333