Request: Track area article scrolling

# Feature Request
Hi, I was advised to open this request after submitting a review on Google Play.
For a long time I've wanted to be able to scroll through an article using the track area rather than the touch screen or volume buttons. Looking at the code I can't work out why this isn't already happening though. If it were a MotionEvent it seems like it would be handled by `dispatchTouchEvent` passthrough, so maybe it's something like `KeyEvent.KEYCODE_DPAD_DOWN` and needs handling the same as the volume buttons rather than passed on to `dispatchKeyEvent`. I'm happy to contribute in whatever way I can but while I know Java, I don't know Android API.

# Environment details
- **wallabag app version**: 2.2.2
- **wallabag app installation source** (e.g. Gplay, F-Droid, manual): GPlay
- **Android OS version**: 8.1.0
- **Android ROM** (e.g. stock, LineageOS, SlimRom,…): stock
- **Android hardware**: Blackberry KeyONE