"fetch most recent file version" in background

[schaarsc]

Actual behaviour

• before a file is shown, the app checks if there is a new version on the server.
• during the check only an animated circle is displayed

Expected behaviour

• if a local version is available, show the cached/local version and check for updates in the background
• provide some visual feedback about the ongoing check e.g. a border around the image or an animated circle in one of the corners
• update display if changes are detected

Steps to reproduce

see #2869 or #3041 or

1. connect to a slow network
2. display image
3. change device orientation, the app will check again before showing the image.

Environment data

Android version: 7 Stock or customized system: Stock Nextcloud app version: 3.3.0 Nextcloud server version: 14

[nextcloud-android-bot]

GitMate.io thinks possibly related issues are https://github.com/nextcloud/android/issues/3041 (App tries to "fetch most recent version of the file" while VPN is in waiting for usable network mode), https://github.com/nextcloud/android/issues/2263 (Too many queries in the background?), https://github.com/nextcloud/android/issues/1869 (New background for NC13), https://github.com/nextcloud/android/issues/1797 (Pre-fetching next images ...), and https://github.com/nextcloud/android/issues/174 (NPE in current Version).