Fix photos not updated when mediaList prop changes

[riwu]

Since props.mediaList is internalised as state in the root component, the library will ignore changes in props.mediaList, which will break use cases such as adding/deleting photos.

Solution: Move the selected state from the root component to the Photo component.

Also avoid using index as the key for the FlatList as it breaks the use cases mentioned above.

[halilb]

This is a great fix. Thank you @riwu! And the PR looks clean. I should just make sure this doesn't break any corner cases.

And I have a favor to ask about this. Could you add a showcase about updated mediaList in the example project?

[riwu]

Added deletion to the grid example. Deletion doesn't work very intuitively for full screen mode though (user has to navigate to next photo after deleting), might want to switch FlatList for something simpler for the FullScreenContainer component.

[jothikannan]

@halilb this would be great if we have the delete in full screen too. I hope you will make it work.

[halilb]

It's working great for the grid view @riwu. Thanks again!

It has some problems in full screen mode as you said. So I merged this PR and #87 to track that down. I hope I can find some free time to investigate that.