merge layout areas

[DavidBruchmann]

Each layout area has unique layout areas. Even if different layout areas have the same key, they are still different.
Therefore reduce the layout areas and provide only one for each key.
This makes switching between layouts much easier as the areas never have to be adjusted.

This can be achieved by changing the TCA, so that existing layout areas can be chosen instead of creating new ones. Technically the task is related to #5.

[DavidBruchmann]

Take care on the icons, it was one problem why this could be challenging