Viewing plaintext files from within Cryptomator

We've already implemented #38 that makes plaintext files (`.txt`) somewhat special in Cryptomator. However, it's currently not possible just to view a plaintext file.

Maybe we could reuse the "plaintext editing" activity for viewing purposes. Or maybe we could even remove the "edit" action in the "browse files" activity and add an option to edit the file in the new "viewing plaintext" activity. Something like a toggle between view and edit mode.

Related community topic: https://community.cryptomator.org/t/text-viewer-for-cryptormator-android-app/169