Merging of input files (App)

[abraemer]

Enable users to merge a second file into the currently open file.

• Add a new menu entry "Merge" with sub-menu entries for supported file types
  • These menu entries are disabled when no file is open
• Clicking on the menu item opens a dialog similar to the import dialog where the user can select the file to merge
  • The explanatory text should warn that this operation modifies the currently open .opossum file and cannot be undone
  • Clicking on the "Merge" button in this dialog should trigger the files to be merged and the new .opossum file to be opened
  • If there are unsaved changed, the NotSavedPopup should be shown before opening the merge dialog