sound slider ignores mouse input if mouse goes out of the slider zone

[Lionel Martin]

When setting the sound, if you move your mouse out of the slider zone, the slider should still move accordingly to X position and stop moving when releasing the button.
The current behaviour is that the slider ignores mouse movement if mouse goes out of the slider zone.
So if by mistake you clickdown, try to drag but go out of the slider zone (up or down), release the mouse button, and reenter the slider zone, the slider will still follow the mouse even if you released the click.

This is also true in a lot of applications such as vlc I think but it's very annoying in this menu as it's used quite often.

[Lars Karlitski (larsu)]

Similar to bug #538493 (not really a duplicate, but the same fix will apply to both bugs).