Implement "remove" functionality for CollapsingState

[dmackdev]

Is your feature request related to a problem? Please describe.
I am using CollapsingState for more granular control for managing collapsing headers over using the CollapsingHeader widget directly. This struct contains an state: InnerState field which is persisted in memory. Currently, I believe it is only possible to load and store the persisted InnerState, and it is not possible to remove it from memory:

• CollapsingState does not currently provide a method to remove InnerState,
• InnerState has the visibility of pub(crate), so it cannot be used to remove an IdTypeMap entry directly via generics.

I have a particular use case for being able to remove InnerState from memory, and I think it would be useful in general, given that it is possible to load/save this state.

Describe the solution you'd like
Implement a remove method for CollapsingState to remove the associated InnerState from memory.

Describe alternatives you've considered
Resetting memory is possible, but this would affect all widgets rather than just a single CollapsingState.