Add share image functionality

[ramosmauricio]

I added a share image functionality, when opening a photo we will now have two options, download and share.

the file is downloaded to the temporary directory and then shared with share_plus package.

thanks!

[jonjomckay]

Thank you! I cleaned it up a bit before merging - I hope you don't mind.

I swapped the XFile usage to use the factory method that takes in a byte array, rather than having to download the file to a temporary location, and also wrapped the button in an AsyncButtonBuilder so the user has some feedback that the file is being downloaded before sharing.