[WIP] Completion for events --filter

[albers]

Adds completion for the docker events --filter family.

@thaJeztah PTAL. This is in an early stage (only some filters implemented yet), and I would like to get early feedback whether I'm on the right track here.

For ContainerNames and NetworkNames I reuse existing completion functions in a hacky way. I think we will hit more use cases like this, and a better solution would be appropriate.

Therefore I propose to pull out helper functions that just return the names (which I need here) from the existing helper functions. Now I run into naming issues. Using the network example, the new function should be named NetworkNames, which forces me to rename the existing one to something like CompleteNetworkNames or NetworkNameCompletions.

To stay consistent, this would mean other completion helpers should be renamed as well, e.g. EnvVarNames -> CompleteEnvVarNames and VolumeNames -> CompleteVolumeNames.

WDYT?