Add a new API for Apps Browser to use

[Jacalz]

The old deprecated custom CLI command interface that was exposed as an API (this one https://github.com/fyne-io/fyne/blob/master/cmd/fyne/commands/command.go) has been removed from cmd/fyne tool here. We should introduce a new, more sensible, API for use in https://github.com/fyne-io/apps.

[Jacalz]

Hmm. This might actually not be a blocker now that I think about it. The apps browser can still use the deprecated API until we have added a new API. Nothing from the new commands here is exposed.