Show save dialog before open/import/merge/export

[PhilippMa]

Is your feature request related to a problem? Please describe.
Open/import/merge/export actions can lead to changes being lost or otherwise incorrect results when they are executed while the currently open file has unsaved changes.

Describe the solution you'd like
The NotSavedPopup should be opened when starting any of the above workflows. If the user clicks on "Discard and Proceed" then the workflow continues as before, if the user clicks cancel then the workflow is cancelled.

Describe alternatives you've considered
Another option would be to directly give the user the option to save their changes from this dialog. However, the currently existing Popups in OpossumUI do not yet support this feature.