Handle media sending failures gracefully

[Fohroer]

Expected behavior

Actual button inside program and in notifications bar to cancel sending.

Actual behavior

If you are in zone with unstable connection or haven't got Internet connection with reasonable speed to send photo program tries to send photo for hours.

Steps to reproduce

1. Try to send big photo to yourself when your Internet provider cut your speed to Edge

Environment

Kontalk version: latest from F-droid

Android version: 6.0

Device model: Vernee Thor

[daniele-athome]

That's reasonable. Probably something like "retry X times and then give up" would be best. And of course a cancel button in the notification.