Concept: Describe the workflow of porting Publisher updates to the generated user apps

@wolfgang @joern @lasse How should general Publisher updates reach the generated user apps?

Example CSS update:
I want to adapt css changes to (all) my generated app(s), but there is currently no workflow described or even possible.
Just copy&pasting the file or directory and to at least move the updates manually in exide is not a very convenient and also an error-prone way of updating.

I think it is a possible use case to change the layout of the main app and wanting it to affect all generated apps.

Related to #56, #36 (closed)