Add option to keep state between sheet displays

[MatFl]

Due to the asynchronous handling when dismissing/showing the view it is impossible to have listeners set after re-showing.

First time:

• add state change listener
• show view -> listener gets called

Second time:

• add listener
• show view (it is already showing) -> calls dismiss and returns
• gets dismissed -> removes all listeners (including new ones)
• runnable shows view again -> no listeners

Adding the listeners after calling show also doesn't work (because the runnable to show again is asynchronously called from the dismiss animation callback)

My idea would be to store a local copy of the listeners in the dismiss method and clean the global fields. Then from the animation listener:

• When it wasn't cancelled call the local listeners.
• If it was cancelled put the local listeners back into the global fields. (To remain the state before the dismiss)

[ZacSweers]

This is a fair point, and it currently does this on purpose. This was initially sensible, bu we later added the option to dismiss and re-show in a single call, which kind of threw a wrench in things. We'll add something in the future to allow the user to specify keeping listeners

[MatFl]

I'm not sure keeping the whole state (and thus the listeners) is the right way. In my case, each sheet comes with its own listeners. The old listeners are only valid for the old sheet. Keeping all the listeners could turn out really messy.