Add the ability to clear a set of rad inbox items by a notifications state.

This is achieved by introducing a field to the Clear operation to optionally clear by a state. When rad inbox clear is called, it will check if the --by-state option is used, if not then the behaviour is as before, otherwise it will get all notification ids matching the given state (and Mode) before removing all those notifications.

Changes:

• Rebased
• Avoid collecting into a Vec and then pushing to another Vec by using closures and filter_map.