'Device is disconnected' while viewing a file

[Mikhail Lopatkin]

"Device is disconnected" message pops up after device being disconnected even if a file is open and not live adb connection.

Steps to reporduce:

1. Start application with device connected
2. Wait for device log to appear
3. Open a file
4. Disconnect device

Expected result: nothing should happen

Actual result: "Device is disconnectected" message appears

[Mikhail Lopatkin]

Rewrite disconnect notification handling a bit

Instead of tracking a device, track the data source instead. It fixes a discrepancy when the data source is already gone but the disconnect handler still tracks the device.

Fixes #96

→ <<cset 2120d80cc104>>