Add option to ignore missing keys or set them to 0

[jpoeppel]

Currently they are always ignored.

[jpoeppel]

Maybe create a popup dialog where advanced settings can be made such as this, in order to prevent cluttering the gui even more.

[jpoeppel]

Commit ca46d81 added a command line argument to define the behaviour (either ignore or use default value X) for timelines, globally. Maybe we want to have more fine grained control over this (e.g. different behaviour for each channel) so I'm not closing this yet.