fixes compilation issues with last released lmdb++

[skip-ci]

This also removes an ugly hack, that was needed before.

This requires Qt 5.9 (to calculate overshoot).
The default scroll behaviour of list views has far too much inertia.
This should make scrolling feel more like scrolling the other scroll
areas of nheko.

Add file encryption / decryption support

It is a bit of a hack, but it works...