Ask for conformation before performing actions that cannot be undone

[jeroens]

When you close the model after deleting groups, there is no prompt to save the model, implying it has already been saved - perhaps there should be a warning to users that group deletion cannot be undone?

This are separate issues.

1. When deleting any data, the application should ask for confirmation
2. When items have been added or removed, there is no need to save the model. Data additions and removals are instantaneous (just like in EwE5)

[administrator]

related to #19

[jeroens]

In progress; implemented for Groups

[jeroens]

All occurrences are now handled.

[jeroens]

All occurrences have been treated.