add the option to only load apps of a specific type

apps/files_archive/appinfo/info.xml

apps/files_encryption/appinfo/info.xml

apps/files_remote/appinfo/info.xml

apps/files_sharing/appinfo/info.xml

files/ajax/download.php

files/ajax/list.php

files/ajax/mimeicon.php

files/webdav.php

lib/app.php

lib/base.php