- rename getAllCustomFields() to getCustomFields(), add return type
- rename $tagQuery to $customQueryPart
- rename setTag, getTag() to setActiveTag(), getActiveTag()
- rename setAssetCollection(), getAssetCollection() to
  setActiveAssetCollection(), getActiveAssetCollection()

This renames some variables and fixes code style issues.

# Conflicts:
#	Classes/Service/CantoClient.php
#	Configuration/Settings.yaml

When Canto needs to be accessed e.g. from the CLI, no Neos user is
active and thus no access token can be used. For those cases, a switch
to client credentials mode is now possible.

Note: With this, the need for appId and appSecret in CantoAssetSource
is already back. :)

# Conflicts:
#	README.md

Using the Canto API from CLI is possible with client credentials mode
by now.

# Conflicts:
#	README.md

# Conflicts:
#	README.md