Improve FileActions JS to allow for custom rendering

This improves the OCA.Files.FileActions class to support passing a
"render" function in the action object.

The default function "_defaultRenderFunction" is used by default and
renders actions in the usual actions container.

Moved "Rename" and "Delete" to custom render functions.
Merge pull request #12394 from owncloud/files-fileactionsimprovements

Improve FileActions JS to allow for custom rendering