"fetch most recent file version" in background

### 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