Show diff in program zexport_gui

[bunysae]

Rexport in an existing bundle:
Table content can differ (exported, actual). Mark these tables in program zexport_gui and let the user choose for each table conjunction, if the content should be exported again or if it should be keeped.

[germanysources]

This function is nearly complete. Just one issue:
When a bundle is read and the name of the bundle or the tdc-variant is changed, it should set all rows to "overwrite"