Improve DebugCommands

[gennadigennadigennadi]

Currently the DebugCommands require the depfile as an argument. Instead of having it required I would like to make it an optional Option. This is also the way the AnalyseCommand handles the depfile.

Additionally the layer argument could also become optional, instead we could just return all layers.

[patrickkusebauch]

@dbrumann @gennadigennadigennadi This also requires a change in the documentation.

Also, the implementation is a breaking change just to be aware.