Total time of the playlist is incorrectly calculated when I add two or more songs at once

[glaubersm]

Describe the bug
Total time of the playlist is incorrectly calculated when I add two or more songs at once. Strawberry takes only the first song into account.

To Reproduce
Steps to reproduce the behavior.

1. open Strawberry
2. create a new playlist
3. drag two or more audio files from the file manager and drop them in the playlist created in the step 2

Expected behavior
Strawberry shoud display the total time of the songs added to playlist beside the seek bar on bottom

Screenshots:
If applicable, add screenshots to help explain your problem.A clear and concise description of what you expected to happen.

System Information:

• Operating system: Arch Linux
• Strawberry Version: 1.0

Additional context
the bug also occurs if the songs are added via File menu > Open file...