add a "Remove non-EnMAP-Box layer from project" button

While #812 isn’t fixed (and maybe it can’t be fixed), we need somethink to clean up the project layer list.

For example, starting with a fresh EnMAP-Box and just import a PRISMA product, will produce duplicates in QgsMapLayerComboBox widgets:

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It is proposed to have at leased a “developer” option for cleaning up the duplicates:

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