Opening a cuesheet adds it to the current cuesheet

[NeoCoderMatrix86]

Describe the bug
Having a cuesheet opened and opening another cuesheet just adds the tracks to the current cuesheet.

To Reproduce
Steps to reproduce the behavior:

1. Have a cuesheet with tracks
2. Click on 'File' -> 'Open' and open a local cuesheet file
3. See the tracks beeing attached

Expected behavior
If there are unsaved changes, I would assume to have a dialog asking if I really want to import the cuesheet. If I click yes, the current cuesheet should be overwritten by the import. The same question should come up when opening an project file.