Enhance import workflow for non-.opossum files

[PhilippMa]

Importing non-.opossum files should no longer be possible through "Open File", but should rather be handled by a new import action that improves on the workflow which is currently triggered when opening non-.opossum files through "Open File".

• Restrict "Open File" to only allow .opossum files
• Add new "Import" menu entry to the "File" menu, with a submenu entry for the outdated opossum file format
• Create a dialog that is opened when the submenu entry is clicked. The dialog should allow to select an input file and specify where the derived .opossum file should be saved. It should also provide a brief explanation about the import process.
• When the user clicks to proceed with the import, the creation and loading of the .opossum file should proceed as in the current version.