Shared: Added function insertMedia which is an alias for M.insert

I think ultimately it is better design to make MediaBag a newtype and interact with it through a fixed interface. Firstly this means you don't have to know about the IR and import Data.Map, secondly it will be easy to perform normalisation on the contents in the future if we need to. (Also, we can defined a nicer Show instance for it which doesn't spam the console when debugging.)