Hello Markus Berchtold,
Thank you for getting back to me.
Yes, you are right that in order to do this you have to drag a 100% layout into the contentplaceholder and then set the permissions. However, while researching this functionality we found some suspicious behavior related to adding widgets to the layout and logged it as a bug. You can check it in PITS here
The settings depend on the span of control you would like to give to your users, and the roles they belong to. In my scenario I have one administrator who has global contorl, so I have restricted all the permissions to Administrators only.
Regarding your last question, you might still use master pages if you wish to do some fine-tuning to your template, but yes, you do need to add layout elements in Sitefinity to be able to set their permissions accordingly.
the Telerik team