Make it possible to opt out of autosaveEnabled in editor extensions

[pascalwengerter]

Is your feature request related to a problem? Please describe.

I've started owncloud/web-extensions#43 and would like to use the existing extension system, most importantly the AppTopBar and the Save button/action there, but without the autosaving - you don't want to implicitly overwrite your original media files while only being halfway done

Describe the solution you'd like

Give extensions a configuration option to opt out and respect it in the AppWrapper