Unsaved popup not shown when opening a file via electron menu or hotkey.

[alexzurbonsen]

Describe the bug
This bug describes the other half of the bug mentioned in #2234.
There are two ways to open a new file:

1. open file button in the top bar

2. via the menu option and hotkey
   In both cases the unsaved popup warning should be shown if necessary, before opening a new file.

3. is taken care of in fix: warn user of unsaved changes if file is opened via button in top bar #2378.

To Reproduce

Steps to reproduce the behavior:

1. Open a file
2. open/create an attribution
3. modify the content, without saving
4. Choose open file option from Electron/Opossum menu (or use respective hotkey)
5. No unsaved popup is shown

Expected behavior
The unsaved pop up should be shown, as well as subsequent warnings that apply if a change is saved (the latter part should already be handled approriately in #2378.