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.

Note: this change will be needed when adding the "Favorite" action (mostly to avoid additional ugly hard-codings...)

Please review @MorrisJobke @icewind1991 @schiesbn @th3fallen 
The idea was conditionally disabled actions.
In such case the implementer of the "render()" function must make it return false/null and not render anything to disable the action based on the context condition.